drm/i915: Do not call intel_crtc_disable if the crtc is already disabled.

When resuming with dpms off, the following warn can happen:

[  118.334082] ------------[ cut here ]------------
[  118.334105] WARNING: CPU: 2 PID: 2274 at drivers/gpu/drm/i915/intel_display.c:6319 __intel_set_mode+0xae5/0xb90 [i915]()
[  118.334106] WARN_ON(!crtc->state->enable)
[  118.334137] Modules linked in: i915
[  118.334139] CPU: 2 PID: 2274 Comm: kworker/u16:117 Not tainted 4.2.0-rc2-fixes+ #4148
[  118.334140] Hardware name: LENOVO 2349AV8/2349AV8, BIOS G1ETA5WW (2.65 ) 04/15/2014
[  118.334144] Workqueue: events_unbound async_run_entry_fn
[  118.334147]  ffffffffc017eef0 ffff8800ada93998 ffffffff817aa62a 0000000080000001
[  118.334149]  ffff8800ada939e8 ffff8800ada939d8 ffffffff810807e1 ffff8800ada939c8
[  118.334151]  ffff8800cea3b3d8 0000000000000000 ffff8800ad86b008 ffff880117705668
[  118.334151] Call Trace:
[  118.334155]  [<ffffffff817aa62a>] dump_stack+0x4f/0x7b
[  118.334157]  [<ffffffff810807e1>] warn_slowpath_common+0x81/0xc0
[  118.334158]  [<ffffffff81080861>] warn_slowpath_fmt+0x41/0x50
[  118.334173]  [<ffffffffc0120375>] __intel_set_mode+0xae5/0xb90 [i915]
[  118.334188]  [<ffffffffc0121312>] ? intel_modeset_compute_config+0x52/0xb40 [i915]
[  118.334191]  [<ffffffff8144de53>] ? drm_atomic_set_fb_for_plane+0x63/0x80
[  118.334205]  [<ffffffffc01269d9>] intel_set_mode+0x29/0x60 [i915]
[  118.334219]  [<ffffffffc012730a>] intel_crtc_restore_mode+0x13a/0x1f0 [i915]
[  118.334232]  [<ffffffffc0101160>] ? gen6_write16+0x250/0x250 [i915]
[  118.334246]  [<ffffffffc01283ec>] intel_modeset_setup_hw_state+0x89c/0xcd0 [i915]
[  118.334248]  [<ffffffff8137d260>] ? pci_pm_thaw+0x90/0x90
[  118.334255]  [<ffffffffc00ac11b>] i915_drm_resume+0xcb/0x160 [i915]
[  118.334262]  [<ffffffffc00ac1d2>] i915_pm_resume+0x22/0x30 [i915]
[  118.334263]  [<ffffffff8137d2c3>] pci_pm_resume+0x63/0xa0
[  118.334266]  [<ffffffff81467550>] dpm_run_callback+0x70/0x420
[  118.334267]  [<ffffffff81467cbd>] device_resume+0x9d/0x1c0
[  118.334269]  [<ffffffff814673d0>] ? initcall_debug_start+0x60/0x60
[  118.334270]  [<ffffffff81467dfc>] async_resume+0x1c/0x50
[  118.334271]  [<ffffffff810a6a94>] async_run_entry_fn+0x34/0xd0
[  118.334273]  [<ffffffff8109d4ad>] process_one_work+0x1dd/0x7e0
[  118.334275]  [<ffffffff8109d41a>] ? process_one_work+0x14a/0x7e0
[  118.334276]  [<ffffffff8109daf9>] worker_thread+0x49/0x450
[  118.334278]  [<ffffffff8109dab0>] ? process_one_work+0x7e0/0x7e0
[  118.334280]  [<ffffffff810a3cb9>] kthread+0xf9/0x110
[  118.334282]  [<ffffffff810a3bc0>] ? insert_kthread_work+0x90/0x90
[  118.334284]  [<ffffffff817b414f>] ret_from_fork+0x3f/0x70
[  118.334286]  [<ffffffff810a3bc0>] ? insert_kthread_work+0x90/0x90
[  118.334287] ---[ end trace 01f2cf6371b82d7a ]---

This warn is harmless, and can be fixed by not calling intel_crtc_disable when
the crtc is already disabled.
Reported-and-Tested-by: Jörg Otte <jrg.otte@gmail.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -6315,9 +6315,6 @@ static void intel_crtc_disable(struct drm_crtc *crtc)
 	struct drm_connector *connector;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
-	/* crtc should still be enabled when we disable it. */
-	WARN_ON(!crtc->state->enable);
-
 	intel_crtc_disable_planes(crtc);
 	dev_priv->display.crtc_disable(crtc);
 	dev_priv->display.off(crtc);
@@ -12591,7 +12588,8 @@ static int __intel_set_mode(struct drm_crtc *modeset_crtc,
 			continue;
 
 		if (!crtc_state->enable) {
-			intel_crtc_disable(crtc);
+			if (crtc->state->enable)
+				intel_crtc_disable(crtc);
 		} else if (crtc->state->enable) {
 			intel_crtc_disable_planes(crtc);
 			dev_priv->display.crtc_disable(crtc);