drm/plane-helper: Provide DRM_PLANE_NON_ATOMIC_FUNCS initializer macro

Provide DRM_PLANE_NON_ATOMIC_FUNCS, which initializes plane functions
of non-atomic drivers to default values. The macro is not supposed to
be used in new code, but helps with documenting and finding existing
users.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Lyude Paul <lyude@redhat.com> # nouveau
Link: https://patchwork.freedesktop.org/patch/msgid/20220909105947.6487-5-tzimmermann@suse.de