drm/udl: Various improvements to the connector

Add style fixes, better error handling and reporting, and minor
clean-up changes to the connector code before moving the code to
the rest of the modesetting pipeline.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-5-tzimmermann@suse.de