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>

drivers/gpu/drm/i915/intel_display.c

@@ -4413,7 +4413,8 @@ static void intel_increase_pllclock(struct drm_crtc *crtc, bool schedule)
 		DRM_DEBUG_DRIVER("upclocking LVDS\n");
 
 		/* Unlock panel regs */
-		I915_WRITE(PP_CONTROL, I915_READ(PP_CONTROL) | (0xabcd << 16));
+		I915_WRITE(PP_CONTROL, I915_READ(PP_CONTROL) |
+			   PANEL_UNLOCK_REGS);
 
 		dpll &= ~DISPLAY_RATE_SELECT_FPA1;
 		I915_WRITE(dpll_reg, dpll);
@@ -4456,7 +4457,8 @@ static void intel_decrease_pllclock(struct drm_crtc *crtc)
 		DRM_DEBUG_DRIVER("downclocking LVDS\n");
 
 		/* Unlock panel regs */
-		I915_WRITE(PP_CONTROL, I915_READ(PP_CONTROL) | (0xabcd << 16));
+		I915_WRITE(PP_CONTROL, I915_READ(PP_CONTROL) |
+			   PANEL_UNLOCK_REGS);
 
 		dpll |= DISPLAY_RATE_SELECT_FPA1;
 		I915_WRITE(dpll_reg, dpll);