drm/udl: Fix missing error code in udl_handle_damage()

If udl_get_urb() fails then this should return a negative error code
but currently it returns success.

Fixes: 798ce3fe

 ("drm/udl: Begin/end access to imported buffers in damage-handler")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201113101502.GD168908@mwanda