virgl: Use virgl_staging_mgr

virgl: Use virgl_staging_mgr

Use an instance of virgl_staging_mgr instead of u_upload_mgr to handle
the staging buffer. This removes the need to track the availability
of the staging manager, since virgl_staging_mgr can handle concurrent
active allocations.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>