ACPI / video: Fix applying indexed initial brightness value.

The value initially read via _BQC also needs to be offset by 2 to
compensate for the first 2 special items in _BCL. Introduce a helper
function that does the BQC-value-to-level conversion in order to not
needlessly duplicate code.

Signed-off-by: Danny Baumann <dannybaumann@web.de>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>