The udl driver is the only caller of drm_plane_helper_atomic_check().
Move the function into the driver. No functional changes.

v2:
	* fix documenation (Sui)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20231204090852.1650-2-tzimmermann@suse.de