drm/udl: Unregister device before cleaning up on disconnect

Disconnecting a DisplayLink device results in the following kernel
error messages

[ 93.041748] [drm:udl_urb_completion [udl]] *ERROR* udl_urb_completion - nonzero write bulk status received: -115
[ 93.055299] [drm:udl_submit_urb [udl]] *ERROR* usb_submit_urb error fffffffe
[ 93.065363] [drm:udl_urb_completion [udl]] *ERROR* udl_urb_completion - nonzero write bulk status received: -115
[ 93.078207] [drm:udl_submit_urb [udl]] *ERROR* usb_submit_urb error fffffffe

coming from KMS poll helpers. Shutting down poll helpers runs them
one final time when the USB device is already gone.

Run drm_dev_unplug() first in udl's USB disconnect handler. Udl's
polling code already handles disconnects gracefully if the device has
been marked as unplugged.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: b1a981bd5576 ("drm/udl: drop drm_driver.release hook")
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.8+
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250303145604.62962-2-tzimmermann@suse.de