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

The PRM does not describe the relevant bits, but Linux's i915 driver
handles these bits the same way for both Ironlake and Sandy Bridge.

Change-Id: Ice7412e335752bd7e297ad50f685effcefbd41d2
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45036
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/src/mainboard/lenovo/x201/devicetree.cb b/src/mainboard/lenovo/x201/devicetree.cb
index d374cec..6b6543d 100644
--- a/src/mainboard/lenovo/x201/devicetree.cb
+++ b/src/mainboard/lenovo/x201/devicetree.cb
@@ -9,7 +9,7 @@
 	register "gpu_dp_d_hotplug" = "0x06"
 
 	# Enable Panel as LVDS and configure power delays
-	register "gpu_panel_port_select" = "0"			# LVDS
+	register "gpu_panel_port_select" = "PANEL_PORT_LVDS"
 	register "gpu_panel_power_cycle_delay" = "3"
 	register "gpu_panel_power_up_delay" = "250"
 	register "gpu_panel_power_down_delay" = "250"