st/omx/bellagio: Rename st and target directories

v2: Refactor out screen functions to st/omx

Allows to keep all the code under st/omx (st/omx/tizonia and
st/omx/bellagio).
Reverts targets/omx_bellagio to omx as additions to existing files
is enough to compile for both bellagio and tizonia.

* autotools changes:
  --enable-omx -> --enable-omx-bellagio

* meson changes:
  -Dgallium-omx=false -> -Dgallium-omx=disabled
  -Dgallium-omx=true  -> -Dgallium-omx=bellagio

Acked-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Julien Isorce <julien.isorce@gmail.com>
diff --git a/meson_options.txt b/meson_options.txt
index 7fafe2d..f5705c2 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -90,9 +90,9 @@
 option(
   'gallium-omx',
   type : 'combo',
-  value : 'auto',
-  choices : ['auto', 'true', 'false'],
-  description : 'enable gallium omx bellagio state tracker.',
+  value : 'bellagio',
+  choices : ['disabled', 'bellagio'],
+  description : 'enable gallium omx state tracker.',
 )
 option(
   'omx-libs-path',