drm/udl: Remove udl_simple_display_pipe_mode_valid()

Remove the empty function udl_simple_display_pipe_mode_valid() and
let simple-KMS helpers accept the modes. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-7-tzimmermann@suse.de