drm/fsl-dcu: use PTR_ERR_OR_ZERO() to simplify the code

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR.

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>