LinuxKPI: Add acpi_video_backlight_use_native()

We assume that backlight (in Linux term) is always "native".

Also stub acpi_video_register_backlight()

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42814