drm/udl: Support DRM hot-unplugging

Add drm_dev_enter() and drm_dev_exit() to the various modesetting
functions that interact with the device. After hot-unplugging the
device, these functions will return early. So far, the udl driver
relied on USB interfaces to handle unplugging of the device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-10-tzimmermann@suse.de