drm/rcar-du: fix return value check in rcar_du_lvdsenc_get_resources()

In case of error, the function devm_ioremap_resource() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR(). Also remove the dev_err call to avoid redundant
error message.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>