asus-wmi: control backlight power through WMI, not ACPI

BugLink: https://bugs.launchpad.net/bugs/1000146
Some h/w that can adjust screen brightness through ACPI functions, but
can't turn on/off the backlight power correctly. So, we list those h/w in
quirks and try to turn on/off the backlight power through WMI.
Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>