drm/i915: add PANEL_UNLOCK_REGS definition

In some cases, unlocking the panel regs is safe and can help us avoid a
flickery, full mode set sequence.  So define the unlock key and use it.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 150400f..c41f945 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -2805,6 +2805,7 @@
 
 #define PCH_PP_STATUS		0xc7200
 #define PCH_PP_CONTROL		0xc7204
+#define  PANEL_UNLOCK_REGS	(0xabcd << 16)
 #define  EDP_FORCE_VDD		(1 << 3)
 #define  EDP_BLC_ENABLE		(1 << 2)
 #define  PANEL_POWER_RESET	(1 << 1)