drm/mediatek: Use ERR_CAST instead of ERR_PTR(PTR_ERR())

Use ERR_CAST inlined function instead of ERR_PTR(PTR_ERR(...)).

drivers/gpu/drm/mediatek/mtk_drm_gem.c:223:9-16: WARNING: ERR_CAST can be used with mtk_gem
Generated by: scripts/coccinelle/api/err_cast.cocci

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: CK Hu <ck.hu@mediatek.com>