Udl's struct udl_connector is an empty wrapper around struct
drm_connector. Remove it. Allocate the connector as part of struct
udl_device and inline the init function into its only caller.

v2:
- fix return value in udl_modeset_init() (Dan)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510154841.11370-6-tzimmermann@suse.de