drm/i915: TLB invalidation with MI_FLUSH_DW requires a post-sync op v3

So store into the scratch space of the HWS to make sure the invalidate
occurs.

v2: use GTT address space for store, clean up #defines (Chris)
v3: use correct #define in blt ring flush (Chris)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Antti Koskipää <antti.koskipaa@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
References: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1063252
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 3745d1d..5af65b8 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -183,6 +183,8 @@
  * The area from dword 0x20 to 0x3ff is available for driver usage.
  */
 #define I915_GEM_HWS_INDEX		0x20
+#define I915_GEM_HWS_SCRATCH_INDEX	0x30
+#define I915_GEM_HWS_SCRATCH_ADDR (I915_GEM_HWS_SCRATCH_INDEX << MI_STORE_DWORD_INDEX_SHIFT)
 
 void intel_cleanup_ring_buffer(struct intel_ring_buffer *ring);