drm/i915: check VBT for supported backlight type

The only supported types are none and PWM. Other values are obsolete or
reserved, don't add them.

Tested-by: Kamal Mostafa <kamal@canonical.com>
Tested-by: Martin <bugs@mrvanes.com>
Tested-by:  <jrg.otte@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>