drm/i915: fix WaDisablePSDDualDispatchEnable on VLV v2

Can prevent a hang when we get to tessellation.  We need to set bit 15
as well for this workaround.

v2: update changelog with accurate info
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -3899,8 +3899,10 @@ static void valleyview_init_clock_gating(struct drm_device *dev)
 		   CHICKEN3_DGMG_REQ_OUT_FIX_DISABLE |
 		   CHICKEN3_DGMG_DONE_FIX_DISABLE);
 
+	/* WaDisablePSDDualDispatchEnable */
 	I915_WRITE(GEN7_HALF_SLICE_CHICKEN1,
-		   _MASKED_BIT_ENABLE(GEN7_PSD_SINGLE_PORT_DISPATCH_ENABLE));
+		   _MASKED_BIT_ENABLE(GEN7_MAX_PS_THREAD_DEP |
+				      GEN7_PSD_SINGLE_PORT_DISPATCH_ENABLE));
 
 	/* Apply the WaDisableRHWOOptimizationForRenderHang workaround. */
 	I915_WRITE(GEN7_COMMON_SLICE_CHICKEN1,