drm/udl: Rename struct udl_drm_connector to struct udl_connector

Remove the _drm_ infix from struct udl_drm_connector and introduce a
macro for upcasting from struct drm_connector. 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/20221006095355.23579-2-tzimmermann@suse.de