cirrus: avoid crash if driver fails to load

If we haven't inited the mm code, don't try and tear it down.

Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/cirrus/cirrus_drv.h

@@ -145,7 +145,7 @@ struct cirrus_device {
 		struct ttm_bo_device bdev;
 		atomic_t validate_sequence;
 	} ttm;
-
+	bool mm_inited;
 };