drm/i915: add panel reset workaround

Ironlake requires that we clear the reset panel bit during power
sequences and restore it afterwards.  Uncondtionally add code to do that
since it should be harmless on SNB+.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/drm/i915/intel_dp.c

@@ -765,6 +765,12 @@ static void ironlake_edp_panel_on (struct drm_device *dev)
 		return;
 
 	pp = I915_READ(PCH_PP_CONTROL);
+
+	/* ILK workaround: disable reset around power sequence */
+	pp &= ~PANEL_POWER_RESET;
+	I915_WRITE(PCH_PP_CONTROL, pp);
+	POSTING_READ(PCH_PP_CONTROL);
+
 	pp |= PANEL_UNLOCK_REGS | POWER_TARGET_ON;
 	I915_WRITE(PCH_PP_CONTROL, pp);
 
@@ -773,7 +779,9 @@ static void ironlake_edp_panel_on (struct drm_device *dev)
 			  I915_READ(PCH_PP_STATUS));
 
 	pp &= ~(PANEL_UNLOCK_REGS | EDP_FORCE_VDD);
+	pp |= PANEL_POWER_RESET; /* restore panel reset bit */
 	I915_WRITE(PCH_PP_CONTROL, pp);
+	POSTING_READ(PCH_PP_CONTROL);
 }
 
 static void ironlake_edp_panel_off (struct drm_device *dev)
@@ -782,6 +790,12 @@ static void ironlake_edp_panel_off (struct drm_device *dev)
 	u32 pp;
 
 	pp = I915_READ(PCH_PP_CONTROL);
+
+	/* ILK workaround: disable reset around power sequence */
+	pp &= ~PANEL_POWER_RESET;
+	I915_WRITE(PCH_PP_CONTROL, pp);
+	POSTING_READ(PCH_PP_CONTROL);
+
 	pp &= ~POWER_TARGET_ON;
 	I915_WRITE(PCH_PP_CONTROL, pp);
 
@@ -790,8 +804,9 @@ static void ironlake_edp_panel_off (struct drm_device *dev)
 			  I915_READ(PCH_PP_STATUS));
 
 	/* Make sure VDD is enabled so DP AUX will work */
-	pp |= EDP_FORCE_VDD;
+	pp |= EDP_FORCE_VDD | PANEL_POWER_RESET; /* restore panel reset bit */
 	I915_WRITE(PCH_PP_CONTROL, pp);
+	POSTING_READ(PCH_PP_CONTROL);
 }
 
 static void ironlake_edp_backlight_on (struct drm_device *dev)