drm/i915: Don't call encoder's get_config unless encoder is active

The SDVO code tries to compare the encoder's and crtc's idea of the
pixel_multiplier. Normally they have to match, but when transitioning
to DPMS off, we turn off the pipe before reading out the pipe_config,
so the pixel_multiplier in the pipe_config will be 0, whereas the
encoder will still have its pixel_multiplier set to whatever value we
were using when the display was active. This leads to a warning
from intel_modeset_check_state().

WARNING: CPU: 1 PID: 2846 at drivers/gpu/drm/i915/intel_sdvo.c:1378 intel_sdvo_get_config+0x158/0x160()
SDVO pixel multiplier mismatch, port: 0, encoder: 1
Modules linked in: snd_hda_codec_idt snd_hda_intel snd_hda_codec snd_hwdep
CPU: 1 PID: 2846 Comm: Xorg Not tainted 3.11.0-rc3-00208-gbe1e8d7-dirty #19
Hardware name: Apple Computer, Inc. Macmini1,1/Mac-F4208EC8, BIOS  MM11.88Z.0055.B03.0604071521 04/07/06
 00000000 00000000 ef0afa54 c1597bbb c1737ea4 ef0afa84 c10392ca c1737e6c
 ef0afab0 00000b1e c1737ea4 00000562 c12dfbe8 c12dfbe8 ef0afb14 00000000
 f697ec00 ef0afa9c c103936e 00000009 ef0afa94 c1737e6c ef0afab0 ef0afadc
Call Trace:
 [<c1597bbb>] dump_stack+0x41/0x56
 [<c10392ca>] warn_slowpath_common+0x7a/0xa0
 [<c103936e>] warn_slowpath_fmt+0x2e/0x30
 [<c12dfbe8>] intel_sdvo_get_config+0x158/0x160
 [<c12c3220>] check_crtc_state+0x1e0/0xb10
 [<c12cdc7d>] intel_modeset_check_state+0x29d/0x7c0
 [<c12dfe5c>] intel_sdvo_dpms+0x5c/0xa0
 [<c12985de>] drm_mode_obj_set_property_ioctl+0x40e/0x420
 [<c1298625>] drm_mode_connector_property_set_ioctl+0x35/0x40
 [<c1289294>] drm_ioctl+0x3e4/0x540
 [<c10fc1a2>] do_vfs_ioctl+0x72/0x570
 [<c10fc72f>] SyS_ioctl+0x8f/0xa0
 [<c159b7fa>] sysenter_do_call+0x12/0x22
---[ end trace 7ce940aff1366d60 ]---

Fix the problem by skipping the encoder get_config() function for
inactive encoders.
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -8269,9 +8269,11 @@ check_crtc_state(struct drm_device *dev)
 
 		list_for_each_entry(encoder, &dev->mode_config.encoder_list,
 				    base.head) {
+			enum pipe pipe;
 			if (encoder->base.crtc != &crtc->base)
 				continue;
-			if (encoder->get_config)
+			if (encoder->get_config &&
+			    encoder->get_hw_state(encoder, &pipe))
 				encoder->get_config(encoder, &pipe_config);
 		}