drm/i915: don't read pp_ctrl_reg if we're suspended

... at edp_have_panel_vdd. Just return false, saying we don't have the
panel VDD since the device is suspended.

We started getting WARNs about this problem since the patch that
started checking if we're suspended while reading registers.

Example backtrace provided by Paulo:

[   63.572201] [drm:hsw_enable_pc8] Enabling package C8+
[   63.581831] [drm:i915_runtime_suspend] Device suspended
[   63.664798] ------------[ cut here ]------------
[   63.664824] WARNING: CPU: 3 PID: 828 at
drivers/gpu/drm/i915/intel_uncore.c:47
assert_device_not_suspended.isra.7+0x32/0x40 [i915]()
[   63.664826] Device suspended
[   63.664828] Modules linked in: ccm fuse ip6table_filter ip6_tables
ebtable_nat ebtables arc4 ath9k_htc ath9k_common ath9k_hw mac80211 ath
cfg80211 iTCO_wdt iTCO_vendor_support x86_pkg_temp_thermal coretemp
microcode i2c_i801 e1000e pcspkr serio_raw lpc_ich ptp pps_core mei_me
mei mfd_core dm_crypt i915 crc32_pclmul crc32c_intel
ghash_clmulni_intel i2c_algo_bit drm_kms_helper drm video
[   63.664867] CPU: 3 PID: 828 Comm: kworker/3:3 Not tainted 3.14.0+ #153
[   63.664869] Hardware name: Intel Corporation Shark Bay Client
platform/WhiteTip Mountain 1, BIOS HSWLPTU1.86C.0133.R00.1309172123
09/17/2013
[   63.664887] Workqueue: events edp_panel_vdd_work [i915]
[   63.664889]  0000000000000009 ffff88009d745c28 ffffffff8167ec6f
ffff88009d745c70
[   63.664895]  ffff88009d745c60 ffffffff8106c8ed ffff880036278000
00000000000c7204
[   63.664900]  ffff88014f2d3040 ffff880036278070 0000000000000001
ffff88009d745cc0
[   63.664905] Call Trace:
[   63.664911]  [<ffffffff8167ec6f>] dump_stack+0x4d/0x66
[   63.664916]  [<ffffffff8106c8ed>] warn_slowpath_common+0x7d/0xa0
[   63.664920]  [<ffffffff8106c95c>] warn_slowpath_fmt+0x4c/0x50
[   63.664926]  [<ffffffff810bd6be>] ? mark_held_locks+0xae/0x130
[   63.664941]  [<ffffffffa00d80d2>]
assert_device_not_suspended.isra.7+0x32/0x40 [i915]
[   63.664956]  [<ffffffffa00d99d2>] gen6_read32+0x32/0x120 [i915]
[   63.664969]  [<ffffffffa00d99a0>] ? gen6_read8+0x120/0x120 [i915]
[   63.664985]  [<ffffffffa0106f8f>] edp_have_panel_vdd+0x3f/0x50 [i915]
[   63.665000]  [<ffffffffa01074e8>] edp_panel_vdd_off_sync+0x58/0x1c0 [i915]
[   63.665004]  [<ffffffff8108a06c>] ? process_one_work+0x18c/0x560
[   63.665018]  [<ffffffffa0107684>] edp_panel_vdd_work+0x34/0x50 [i915]
[   63.665022]  [<ffffffff8108a0d7>] process_one_work+0x1f7/0x560
[   63.665026]  [<ffffffff8108a06c>] ? process_one_work+0x18c/0x560
[   63.665031]  [<ffffffff8108ae2b>] worker_thread+0x11b/0x3a0
[   63.665035]  [<ffffffff8108ad10>] ? manage_workers.isra.21+0x2a0/0x2a0
[   63.665039]  [<ffffffff810916fc>] kthread+0xfc/0x120
[   63.665043]  [<ffffffff81091600>] ? kthread_create_on_node+0x230/0x230
[   63.665048]  [<ffffffff8169082c>] ret_from_fork+0x7c/0xb0
[   63.665052]  [<ffffffff81091600>] ? kthread_create_on_node+0x230/0x230
[   63.665054] ---[ end trace 1250bcc890af9999 ]---
[   63.665060] [drm:edp_panel_vdd_off_sync] Turning eDP VDD off
[   63.665061] ------------[ cut here ]------------

Testcase: igt/pm_pc8
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -314,7 +314,8 @@ static bool edp_have_panel_vdd(struct intel_dp *intel_dp)
 	struct drm_device *dev = intel_dp_to_dev(intel_dp);
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
-	return (I915_READ(_pp_ctrl_reg(intel_dp)) & EDP_FORCE_VDD) != 0;
+	return !dev_priv->pm.suspended &&
+	       (I915_READ(_pp_ctrl_reg(intel_dp)) & EDP_FORCE_VDD) != 0;
 }
 
 static void