drm/vc4: Fix oops when userspace hands in a bad BO.

We'd end up NULL pointer dereferencing because we didn't take the
error path out in the parent.  Fixes igt vc4_lookup_fail test.
Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: d5b1a78a ("drm/vc4: Add support for drawing 3D frames.")
Cc: stable@vger.kernel.org

drivers/gpu/drm/vc4/vc4_gem.c

@@ -573,7 +573,7 @@ vc4_cl_lookup_bos(struct drm_device *dev,
 
 fail:
 	drm_free_large(handles);
-	return 0;
+	return ret;
 }
 
 static int