drm/omap: Use ERR_CAST directly instead of ERR_PTR(PTR_ERR())

We prefer to use ERR_CAST to do so.
The issue is detected with the help of Coccinelle.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>