drm/fsl-dcu: Include <linux/of.h>

Include <linux/of.h> directly to get of_parse_phandle(). Avoids the
proxy include via <linux/backlight.h>

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Alison Wang <alison.wang@nxp.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241011144632.90434-2-tzimmermann@suse.de