drm/ttm: make ttm_tt unbind function return void.

The return value just led to BUG_ON, I think if a driver wants
to BUG_ON here it can do it itself. (don't BUG_ON).

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728040003.20398-1-airlied@gmail.com

include/drm/ttm/ttm_tt.h

@@ -70,7 +70,7 @@ struct ttm_backend_func {
 	 * Unbind previously bound backend pages. This function should be
 	 * able to handle differences between aperture and system page sizes.
 	 */
-	int (*unbind) (struct ttm_tt *ttm);
+	void (*unbind) (struct ttm_tt *ttm);
 
 	/**
 	 * struct ttm_backend_func member destroy