drm/i915: Turn on another required clock gating bit on gen6.

Unlike the previous one, I don't have known testcases it fixes.  I'd
rather not go through the same debug cycle on whatever testcases those
might be.

Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: stable@kernel.org
Signed-off-by: Keith Packard <keithp@keithp.com>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 2b2a764..591eb0ed 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -8154,8 +8154,13 @@
 	 * some amount of runtime in the Mesa "fire" demo, and Unigine
 	 * Sanctuary and Tropics, and apparently anything else with
 	 * alpha test or pixel discard.
+	 *
+	 * According to the spec, bit 11 (RCCUNIT) must also be set,
+	 * but we didn't debug actual testcases to find it out.
 	 */
-	I915_WRITE(GEN6_UCGCTL2, GEN6_RCPBUNIT_CLOCK_GATE_DISABLE);
+	I915_WRITE(GEN6_UCGCTL2,
+		   GEN6_RCPBUNIT_CLOCK_GATE_DISABLE |
+		   GEN6_RCCUNIT_CLOCK_GATE_DISABLE);
 
 	/*
 	 * According to the spec the following bits should be