gma: Add procedure to power up legacy VGA block

Calling this is necessary for VGA text mode on at least Skylake, where
the legacy VGA registers are inaccessible otherwise.

Change-Id: I48ba1738bcc7babd4e666e5266f775dcd06b2a3f
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/21323
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: Nico Huber <nico.h@gmx.de>

common/hw-gfx-gma.adb

@@ -438,6 +438,23 @@ is
 
    ----------------------------------------------------------------------------
 
+   procedure Power_Up_VGA
+   is
+      Fake_Config : constant Pipe_Configs :=
+        (Primary =>
+           (Port        => Analog,
+            Framebuffer => HW.GFX.Default_FB,
+            Mode        => HW.GFX.Invalid_Mode),
+         others =>
+           (Port        => Disabled,
+            Framebuffer => HW.GFX.Default_FB,
+            Mode        => HW.GFX.Invalid_Mode));
+   begin
+      Power_And_Clocks.Power_Up (Cur_Configs, Fake_Config);
+   end Power_Up_VGA;
+
+   ----------------------------------------------------------------------------
+
    function FB_First_Page (FB : Framebuffer_Type) return Natural is
      (Natural (FB.Offset / GTT_Page_Size));
    function FB_Pages (FB : Framebuffer_Type) return Natural is

common/hw-gfx-gma.ads

@@ -80,6 +80,12 @@ is
       Global => (Input => Init_State);
    pragma Warnings (GNATprove, On, "unused variable ""Write_Delay""");
 
+   pragma Warnings (GNATprove, Off, "subprogram ""Power_Up_VGA"" has no effect",
+                    Reason => "Effect depends on the platform compiled for");
+   procedure Power_Up_VGA
+   with
+      Pre => Is_Initialized;
+
    procedure Update_Outputs (Configs : Pipe_Configs);
 
    pragma Warnings (GNATprove, Off, "subprogram ""Dump_Configs"" has no effect",