ACPI / video: get rid of magic numbers and use enum instead

The first two items in the _BCL method response are special:

  - Level when machine has full power
  - Level when machine is on batteries
  - .... actual supported levels go there ....

So this commits adds an enum and uses its descriptive elements
throughout the code, instead of magic numbers.
Signed-off-by: Dmitry Frank <mail@dmitryfrank.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>