drm/i915/bdw: 3D_CHICKEN3 has write mask bits

The workaround to limit SDE poly depth FIFO to 2 is not applied because
3D Chicken-3 mask bit is not set.

WaLimitSizeOfSDEPolyFifo is only for BDW-A and could be removed.
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -5375,7 +5375,7 @@ static void gen8_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(GAMTARBMODE, _MASKED_BIT_ENABLE(ARB_MODE_BWGTLB_DISABLE));
 
 	I915_WRITE(_3D_CHICKEN3,
-		   _3D_CHICKEN_SDE_LIMIT_FIFO_POLY_DEPTH(2));
+		   _MASKED_BIT_ENABLE(_3D_CHICKEN_SDE_LIMIT_FIFO_POLY_DEPTH(2)));
 
 	I915_WRITE(COMMON_SLICE_CHICKEN2,
 		   _MASKED_BIT_ENABLE(GEN8_CSC2_SBE_VUE_CACHE_CONSERVATIVE));