gma hsw+: Add default value for HDMI buffer levels

Add a configuration option to override the default HDMI translation.
It's not exported yet, as we've never seen a non-default value being
used. So this is just to have a common place for the defaults.

Also sets the recommended default value for Broxton as defined in the
PRM.

Change-Id: I59fae0fb4f444d9193a98b6a0edf337ecbba3b62
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/20165
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/common/broxton/hw-gfx-gma-ddi_phy.ads b/common/broxton/hw-gfx-gma-ddi_phy.ads
index 98d0594..fc6cacf 100644
--- a/common/broxton/hw-gfx-gma-ddi_phy.ads
+++ b/common/broxton/hw-gfx-gma-ddi_phy.ads
@@ -38,7 +38,7 @@
      (Port        : Digital_Port;
       Train_Set   : DP_Info.Train_Set);
 
-   type HDMI_Buf_Trans_Range is range 0 .. 9;
+   subtype HDMI_Buf_Trans_Range is DDI_HDMI_Buf_Trans_Range range 0 .. 9;
    procedure Set_HDMI_Signal_Levels
      (Port  : DDI_Phy_Port;
       Level : HDMI_Buf_Trans_Range);