drm/ttm: flip tt destroy ordering.

Call the driver first and have it call the common code cleanup.

This is useful later to fix unbind.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917043040.146575-3-airlied@gmail.com

include/drm/ttm/ttm_tt.h

@@ -155,6 +155,13 @@ void ttm_dma_tt_fini(struct ttm_dma_tt *ttm_dma);
  */
 void ttm_tt_destroy(struct ttm_bo_device *bdev, struct ttm_tt *ttm);
 
+/**
+ * ttm_tt_destroy_common:
+ *
+ * Called from driver to destroy common path.
+ */
+void ttm_tt_destroy_common(struct ttm_bo_device *bdev, struct ttm_tt *ttm);
+
 /**
  * ttm_tt_swapin:
  *