ACPI/video: export acpi_video_get_levels

The acpi_video_get_levels is useful for other drivers, i.e. the
to-be-added int3406 thermal driver, so export it.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>