drm/udl: Switch to atomic suspend/resume helpers

We can use the generic suspend/resume helpers for atomic modesetting.
Switch udl over.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210084905.5570-4-tzimmermann@suse.de