drm/i915: Add thread stall DOP clock gating workaround on Broadwell.

Ben and I believe this will be necessary on production hardware.
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
[danvet: Shuffle lines to group all ROW_CHICKEN writes and add a
cautious comment that this might not be needed on production hw.]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -5050,6 +5050,7 @@
 
 #define GEN8_ROW_CHICKEN		0xe4f0
 #define   PARTIAL_INSTRUCTION_SHOOTDOWN_DISABLE	(1<<8)
+#define   STALL_DOP_GATING_DISABLE		(1<<5)
 
 #define GEN7_ROW_CHICKEN2		0xe4f4
 #define GEN7_ROW_CHICKEN2_GT2		0xf4f4

drivers/gpu/drm/i915/intel_pm.c

@@ -4805,6 +4805,11 @@ static void gen8_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(GEN8_ROW_CHICKEN,
 		   _MASKED_BIT_ENABLE(PARTIAL_INSTRUCTION_SHOOTDOWN_DISABLE));
 
+	/* WaDisableThreadStallDopClockGating:bdw */
+	/* FIXME: Unclear whether we really need this on production bdw. */
+	I915_WRITE(GEN8_ROW_CHICKEN,
+		   _MASKED_BIT_ENABLE(STALL_DOP_GATING_DISABLE));
+
 	/*
 	 * This GEN8_CENTROID_PIXEL_OPT_DIS W/A is only needed for
 	 * pre-production hardware