drm/radeon/kms: allow "invalid" DB formats as a means to disable DB

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/r600_cs.c b/drivers/gpu/drm/radeon/r600_cs.c
index ca87f7a..1119e31 100644
--- a/drivers/gpu/drm/radeon/r600_cs.c
+++ b/drivers/gpu/drm/radeon/r600_cs.c
@@ -764,8 +764,10 @@
 	}
 
 	/* Check depth buffer */
-	if (track->db_dirty && (G_028800_STENCIL_ENABLE(track->db_depth_control) ||
-		G_028800_Z_ENABLE(track->db_depth_control))) {
+	if (track->db_dirty &&
+	    G_028010_FORMAT(track->db_depth_info) != V_028010_DEPTH_INVALID &&
+	    (G_028800_STENCIL_ENABLE(track->db_depth_control) ||
+	     G_028800_Z_ENABLE(track->db_depth_control))) {
 		r = r600_cs_track_validate_db(p);
 		if (r)
 			return r;