drm/i915/gen9: Implement WaDisableDgMirrorFixInHalfSliceChicken5

Move WaDisableDgMirrorFixInHalfSliceChicken5 to gen9_init_workarounds

v2: Added stepping check

v3: Removed unused register bitmap

Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
[danvet: Bikesheds.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index b869f1c..248db51 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -882,6 +882,16 @@
 	WA_SET_BIT_MASKED(GEN8_ROW_CHICKEN,
 			  PARTIAL_INSTRUCTION_SHOOTDOWN_DISABLE);
 
+	if (INTEL_REVID(dev) == SKL_REVID_A0) {
+		/*
+		* WaDisableDgMirrorFixInHalfSliceChicken5:skl
+		* This is a pre-production w/a.
+		*/
+		I915_WRITE(GEN9_HALF_SLICE_CHICKEN5,
+			I915_READ(GEN9_HALF_SLICE_CHICKEN5) &
+			~GEN9_DG_MIRROR_FIX_ENABLE);
+	}
+
 	return 0;
 }