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/Makefile.inc b/common/Makefile.inc
index 11215ee..0583143 100644
--- a/common/Makefile.inc
+++ b/common/Makefile.inc
@@ -36,6 +36,8 @@
 gfxinit-y += hw-gfx-gma-port_detect.ads
 gfxinit-y += hw-gfx-gma-registers.adb
 gfxinit-y += hw-gfx-gma-registers.ads
+gfxinit-y += hw-gfx-gma-transcoder.adb
+gfxinit-y += hw-gfx-gma-transcoder.ads
 gfxinit-y += hw-gfx-gma.adb
 gfxinit-y += hw-gfx-gma.ads
 gfxinit-y += hw-gfx-i2c.ads