gma: Move transcoder setup into own package

Split the transcoder setup out of `Pipe_Setup` into a new package
`Transcoder`. This comes closer to how Intel's manuals describe the
hardware.

Also rework the related constant definitions to make things more
human readable.

Change-Id: Ife0f0d635d87b874d4b713a00ca7a1bec688c672
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/17764
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/common/hw-gfx-gma.adb b/common/hw-gfx-gma.adb
index 0fb4e5d..8d304b6 100644
--- a/common/hw-gfx-gma.adb
+++ b/common/hw-gfx-gma.adb
@@ -135,7 +135,7 @@
 
                   Connectors.Pre_Off (Port_Cfg);
 
-                  Display_Controller.Off (I, Port_Cfg);
+                  Display_Controller.Off (I);
 
                   Connectors.Post_Off (Port_Cfg);
                end if;
@@ -212,7 +212,7 @@
                               Success  => Success);
 
                            if not Success then
-                              Display_Controller.Off (I, Port_Cfg);
+                              Display_Controller.Off (I);
                               Connectors.Post_Off (Port_Cfg);
                            end if;
                         end if;