gma: Implement Ivy Bridge VGA plane workaround

It's a documented requirement, even though it doesn't seem necessary.

Change-Id: Id4f579c1ca34633ee00c771b39e6ff45cdcfbf69
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/17277
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
diff --git a/common/hw-gfx-gma-config.ads.template b/common/hw-gfx-gma-config.ads.template
index 7289dff..ccc5c1a 100644
--- a/common/hw-gfx-gma-config.ads.template
+++ b/common/hw-gfx-gma-config.ads.template
@@ -49,6 +49,7 @@
    Has_Plane_Control       : constant Boolean := CPU >= Skylake;
    Has_DSP_Linoff          : constant Boolean := CPU <= Ivybridge;
    Has_PF_Pipe_Select      : constant Boolean := CPU in Ivybridge .. Haswell;
+   VGA_Plane_Workaround    : constant Boolean := CPU = Ivybridge;
 
    ----- Panel power: -----
    Has_PP_Write_Protection : constant Boolean := CPU <= Ivybridge;