drm/i915: PIPE_CONTROL_TLB_INVALIDATE

This has showed up in several other patches. It's required for the next
context workaround.

I tested this one on its own and saw no differences in basic tests
(performance or otherwise). This patch is relatively likely to cause
regressions, hence why it's split out.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>

drivers/gpu/drm/i915/i915_reg.h

@@ -294,6 +294,7 @@
 #define   DISPLAY_PLANE_B           (1<<20)
 #define GFX_OP_PIPE_CONTROL(len)	((0x3<<29)|(0x3<<27)|(0x2<<24)|(len-2))
 #define   PIPE_CONTROL_CS_STALL				(1<<20)
+#define   PIPE_CONTROL_TLB_INVALIDATE			(1<<18)
 #define   PIPE_CONTROL_QW_WRITE				(1<<14)
 #define   PIPE_CONTROL_DEPTH_STALL			(1<<13)
 #define   PIPE_CONTROL_WRITE_FLUSH			(1<<12)

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -226,6 +226,7 @@ gen6_render_ring_flush(struct intel_ring_buffer *ring,
 	 * impact.
 	 */
 	flags |= PIPE_CONTROL_RENDER_TARGET_CACHE_FLUSH;
+	flags |= PIPE_CONTROL_TLB_INVALIDATE;
 	flags |= PIPE_CONTROL_INSTRUCTION_CACHE_INVALIDATE;
 	flags |= PIPE_CONTROL_TEXTURE_CACHE_INVALIDATE;
 	flags |= PIPE_CONTROL_DEPTH_CACHE_FLUSH;