gma: Add contract to Enable_Output() to rely less on proof inlining

Change-Id: I7bc066b33c969e528c7bcd9328178fac8a37ad21
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/26845
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/common/hw-gfx-gma.adb b/common/hw-gfx-gma.adb
index a5e9dbe..ab4e1e2 100644
--- a/common/hw-gfx-gma.adb
+++ b/common/hw-gfx-gma.adb
@@ -97,6 +97,8 @@
      (Pipe     : in     Pipe_Index;
       Pipe_Cfg : in     Pipe_Config;
       Success  :    out Boolean)
+   with
+      Pre => Pipe_Cfg.Port in Active_Port_Type
    is
       Port_Cfg : Port_Config;
       Scaler_Available : Boolean;