This error path needs an unlock before returning.

Fixes: 604be855 ("drm/rockchip: Add VOP2 driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/YnjZQRV9lpub2ET8@kili