drm/ttm: unbind ttm before destroying node in accel move cleanup

Nouveau makes the assumption that if a TTM is bound there will be a mm_node
around for it and the backwards ordering here resulted in a use-after-free
on some eviction paths.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -635,13 +635,13 @@ int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
 		if (ret)
 			return ret;
 
-		ttm_bo_free_old_node(bo);
 		if ((man->flags & TTM_MEMTYPE_FLAG_FIXED) &&
 		    (bo->ttm != NULL)) {
 			ttm_tt_unbind(bo->ttm);
 			ttm_tt_destroy(bo->ttm);
 			bo->ttm = NULL;
 		}
+		ttm_bo_free_old_node(bo);
 	} else {
 		/**
 		 * This should help pipeline ordinary buffer moves.