meson: Re-add auto option for omx

meson: Re-add auto option for omx

This re-adds the auto option for omx, without it we default to tizonia
and the build fails almost immediately, this is especially obnoxious
those building a driver that doesn't support the OMX state tracker to
begin with.

v2: - Only define OMX_FOO for auto cases if the dependencies are found.
      This fixes building tizonia with auto (Julien, Eric)

CC: Gurkirpal Singh <gurkirpal204@gmail.com>
Fixes: bb5e27fab6087a5c1528a5faf507acce700e883c
       ("st/omx/bellagio: Rename st and target directories")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Jon Turney <jon.turney@dronecode.org.uk> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Tested-by: Julien Isorce <julien.isorce@gmail.com>
Tested-by: Karol Herbst <kherbst@redhat.com> (v1)