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/northbridge/intel/ironlake/chip.h b/src/northbridge/intel/ironlake/chip.h
index c437b72..b2976bc 100644
--- a/src/northbridge/intel/ironlake/chip.h
+++ b/src/northbridge/intel/ironlake/chip.h
@@ -17,7 +17,13 @@
 	u8 gpu_dp_c_hotplug; /* Digital Port C Hotplug Config */
 	u8 gpu_dp_d_hotplug; /* Digital Port D Hotplug Config */
 
-	u8 gpu_panel_port_select; /* 0=LVDS 1=DP_B 2=DP_C 3=DP_D */
+	enum {
+		PANEL_PORT_LVDS = 0,
+		PANEL_PORT_DP_A = 1, /* Also known as eDP */
+		PANEL_PORT_DP_C = 2,
+		PANEL_PORT_DP_D = 3,
+	} gpu_panel_port_select;
+
 	u8 gpu_panel_power_cycle_delay;          /* T4 time sequence */
 	u16 gpu_panel_power_up_delay;            /* T1+T2 time sequence */
 	u16 gpu_panel_power_down_delay;          /* T3 time sequence */