drm/udl: Move clip-rectangle code out of udl_handle_damage()

Computing the clip rectable in a separate helper function makes the
damage-handler code more readable.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206085954.9697-5-tzimmermann@suse.de