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/intel_bios.h

@@ -374,6 +374,9 @@ struct bdb_lvds_lfp_data {
 	struct bdb_lvds_lfp_data_entry data[16];
 } __packed;
 
+#define BDB_BACKLIGHT_TYPE_NONE	0
+#define BDB_BACKLIGHT_TYPE_PWM	2
+
 struct bdb_lfp_backlight_data_entry {
 	u8 type:2;
 	u8 active_low_pwm:1;