drm/i915: There's no need to mask all 3D_CHICKEN bits on SNB

The need to set all of the mask bits for 3D_CHICKEN3 was required
only for pre-production hardware.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4696,9 +4696,8 @@ static void gen6_init_clock_gating(struct drm_device *dev)
 		   GEN6_RCCUNIT_CLOCK_GATE_DISABLE);
 
 	/* WaStripsFansDisableFastClipPerformanceFix:snb */
-	/* Bspec says we need to always set all mask bits. */
-	I915_WRITE(_3D_CHICKEN3, (0xFFFF << 16) |
-		   _3D_CHICKEN3_SF_DISABLE_FASTCLIP_CULL);
+	I915_WRITE(_3D_CHICKEN3,
+		   _MASKED_BIT_ENABLE(_3D_CHICKEN3_SF_DISABLE_FASTCLIP_CULL));
 
 	/*
 	 * Bspec says: