drm/udl: Improve type safety when using struct udl_device

Push upcasts from struct drm_device to struct udl_device outwards
in the call chain; cast earlier and call functions with the upcasted
value. Improves type safety.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://lore.kernel.org/r/20250410105948.25463-4-tzimmermann@suse.de