gma ddi: Don't try to disable non-existent DDI D

Didn't hurt on Haswell-/Broadwell-U, but did lock up Broxton.

Change-Id: I6a47d0fba8cb6e59d8ace1ab64eed6182769a37b
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/18420
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/common/hw-gfx-gma-config.ads.template b/common/hw-gfx-gma-config.ads.template
index b3492f4..5ad6593 100644
--- a/common/hw-gfx-gma-config.ads.template
+++ b/common/hw-gfx-gma-config.ads.template
@@ -156,6 +156,9 @@
       Part_Of => GMA.Config_State;
    Valid_Port : Valid_Per_Port renames Valid_Port_GPU (CPU);
 
+   Last_Digital_Port : constant Digital_Port :=
+     (if Has_DDI_D then DIGI_D else DIGI_C);
+
    ----------------------------------------------------------------------------
 
    type FDI_Per_Port is array (Port_Type) of Boolean;