drm/udl: Convert to drm_crtc_helper_atomic_check()

Use the drm_crtc_helper_atomic_check() helper instead of open-coding the
same operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dcb09c0c7bed1baee39a72caba098a399ce7f063.1689599767.git.geert+renesas@glider.be