ACPI / video: Don't register acpi_video_resume notifier without backlight devices

If we're not going to be registering any backlight devices then
acpi_video_resume is always nop, so don't register it in that case.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>