drm/i915: PIPE_CONTROL TLB invalidate requires CS stall

"If ENABLED, PIPE_CONTROL command will flush the in flight data  written
out by render engine to Global Observation point on flush done. Also
Requires stall bit ([20] of DW1) set."

So set the stall bit to ensure proper invalidation.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Antti Koskipää <antti.koskipaa@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -245,7 +245,7 @@ gen6_render_ring_flush(struct intel_ring_buffer *ring,
 		/*
 		 * TLB invalidate requires a post-sync write.
 		 */
-		flags |= PIPE_CONTROL_QW_WRITE;
+		flags |= PIPE_CONTROL_QW_WRITE | PIPE_CONTROL_CS_STALL;
 	}
 
 	ret = intel_ring_begin(ring, 4);