ACPI: video: Add comment about acpi_video_backlight_use_native() usage

Add a comment explaining that acpi_video_backlight_use_native() MUST
only be used by GPU drivers and that it must NOT be used on other places.

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