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>

drivers/gpu/drm/i915/i915_drv.h

@@ -1308,6 +1308,7 @@ struct intel_vbt_data {
 
 	struct {
 		u16 pwm_freq_hz;
+		bool present;
 		bool active_low_pwm;
 	} backlight;