drm: rcar-du: Fix the return check of of_parse_phandle and of_find_device_by_node

of_parse_phandle and of_find_device_by_node may return NULL
which cannot be checked by IS_ERR.

Fixes: 8de707ae ("drm: rcar-du: kms: Initialize CMM instances")
Signed-off-by: Wang Xiaojun <wangxiaojun11@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Acked-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

[Replace -ENODEV with -EINVAL]

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>