nb/intel/sandybridge: Use an enum for `gpu_panel_port_select`

All boards currently have backlight on either LVDS or eDP.

Change-Id: I878bc7f1ff75a2b82b9556e855aff1d4d03e0268
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45035
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/src/mainboard/apple/macbookair4_2/devicetree.cb b/src/mainboard/apple/macbookair4_2/devicetree.cb
index 6e99b63..f6b0176 100644
--- a/src/mainboard/apple/macbookair4_2/devicetree.cb
+++ b/src/mainboard/apple/macbookair4_2/devicetree.cb
@@ -6,7 +6,7 @@
 	register "gpu_dp_b_hotplug" = "4"
 	register "gpu_dp_c_hotplug" = "4"
 	register "gpu_dp_d_hotplug" = "4"
-	register "gpu_panel_port_select" = "1"
+	register "gpu_panel_port_select" = "PANEL_PORT_DP_A"
 	register "gpu_panel_power_backlight_off_delay" = "2000"
 	register "gpu_panel_power_backlight_on_delay" = "10"
 	register "gpu_panel_power_cycle_delay" = "6"