drm/i915: Ensure the HiZ RAW Stall Optimization is on for Cherryview.

This is an important optimization for avoiding read-after-write (RAW)
stalls in the HiZ buffer.  Certain workloads would run very slowly with
HiZ enabled, but run much faster with the "hiz=false" driconf option.
With this patch, they run at full speed even with HiZ.

Increases performance in OglVSInstancing by about 2.7x on Braswell.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -846,6 +846,11 @@ static int chv_init_workarounds(struct intel_engine_cs *ring)
 			  HDC_FORCE_NON_COHERENT |
 			  HDC_DONOT_FETCH_MEM_WHEN_MASKED);
 
+	/* According to the CACHE_MODE_0 default value documentation, some
+	 * CHV platforms disable this optimization by default.  Turn it on.
+	 */
+	WA_CLR_BIT_MASKED(CACHE_MODE_0_GEN7, HIZ_RAW_STALL_OPT_DISABLE);
+
 	/* Improve HiZ throughput on CHV. */
 	WA_SET_BIT_MASKED(HIZ_CHICKEN, CHV_HZ_8X8_MODE_IN_1X);