samsung-laptop: don't handle backlight if handled by acpi/video

samsung-laptop is not at all related to ACPI, but since this interface
is not documented at all, and the driver has to use it at load to
understand how it works on the laptop, I think it's a good idea to
disable it if a better solution is available.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Matthew Garrett <mjg@redhat.com>