gma: Clear "fence" registers during initialization

These registers are used to mark certain GTT regions as tiled.

Change-Id: Ic2cd61c0c1b42990ed955d7f77a428a2b9dbabd5
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/20601
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/common/hw-gfx-gma-registers.ads b/common/hw-gfx-gma-registers.ads
index 63df8be..2a01aab 100644
--- a/common/hw-gfx-gma-registers.ads
+++ b/common/hw-gfx-gma-registers.ads
@@ -1529,6 +1529,8 @@
        Mask_Unset : Word32;
        Mask_Set   : Word32);
 
+   procedure Clear_Fences;
+
    pragma Warnings (Off, "declaration of ""Write_GTT"" hides one at *");
    procedure Write_GTT
      (GTT_Page       : GTT_Range;