ACPI / i915: ignore firmware requests for backlight change

Some Thinkpad laptops' firmware will initiate a backlight level change
request through operation region on the events of AC plug/unplug, but
since we are not using firmware's interface to do the backlight setting
on these affected laptops, we do not want the firmware to use some
arbitrary value from its ASL variable to set the backlight level on
AC plug/unplug either.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=76491
Link: https://bugzilla.kernel.org/show_bug.cgi?id=77091


Reported-and-tested-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
Reported-and-tested-by: Anton Gubarkov <anton.gubarkov@gmail.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>