drm: gm12u320: Some minor cleanups

3 small cleanups:

1) Drop unused DRIVER_PATCHLEVEL
2) We do not set mode_config.preferred_depth, so instead of passing the
   unset mode_config.preferred_depth to drm_fbdev_generic_setup
   simply pass 0
3) Use __maybe_unused instead of #ifdef CONFIG_PM around the suspend /
   resume functions

Cc: Sam Ravnborg <sam@ravnborg.org>
Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Suggested-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730133857.30778-1-hdegoede@redhat.com