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>
23 files changed