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.adb b/common/hw-gfx-gma.adb
index 387d6bd..94f3ceb 100644
--- a/common/hw-gfx-gma.adb
+++ b/common/hw-gfx-gma.adb
@@ -401,6 +401,7 @@
          Connectors.Post_All_Off;
          PLLs.All_Off;
          Power_And_Clocks.Post_All_Off;
+         Registers.Clear_Fences;
       else
          -- According to PRMs, VGA plane is the only thing
          -- that's enabled by default after reset.