drm/radeon/kms/r7xx: add workaround for hw issue with HDP flush

Use of HDP_*_COHERENCY_FLUSH_CNTL can cause a hang in certain
situations.  Add workaround.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/rv770d.h b/drivers/gpu/drm/radeon/rv770d.h
index fd733f2..b7a5a20 100644
--- a/drivers/gpu/drm/radeon/rv770d.h
+++ b/drivers/gpu/drm/radeon/rv770d.h
@@ -133,6 +133,7 @@
 #define	HDP_NONSURFACE_SIZE				0x2C0C
 #define HDP_REG_COHERENCY_FLUSH_CNTL			0x54A0
 #define	HDP_TILING_CONFIG				0x2F3C
+#define HDP_DEBUG1                                      0x2F34
 
 #define MC_SHARED_CHMAP						0x2004
 #define		NOOFCHAN_SHIFT					12