ACPI / video: force vendor backlight on Lenovo Ideapad Z570

Lenovo Ideapad 570 is a pre-win8 laptop where not using vendor
backlight causes the backlight controls not to work.

Signed-off-by: Stepan Bujnak <stepanbujnak@fastmail.fm>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>