drm/radeon/kms/evergreen: flush hdp cache when flushing gart tlb

Make sure vram changes hit memory.  This mirrors the
6xx/7xx behavior.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/evergreen.c b/drivers/gpu/drm/radeon/evergreen.c
index 4dc5b47..39fa75b 100644
--- a/drivers/gpu/drm/radeon/evergreen.c
+++ b/drivers/gpu/drm/radeon/evergreen.c
@@ -748,6 +748,8 @@
 	unsigned i;
 	u32 tmp;
 
+	WREG32(HDP_MEM_COHERENCY_FLUSH_CNTL, 0x1);
+
 	WREG32(VM_CONTEXT0_REQUEST_RESPONSE, REQUEST_TYPE(1));
 	for (i = 0; i < rdev->usec_timeout; i++) {
 		/* read MC_STATUS */