gallium/u_threaded: implement asynchronous flushes

This requires out-of-band creation of fences, and will be signaled to
the pipe_context::flush implementation by a special TC_FLUSH_ASYNC flag.

v2:
- remove an incorrect assertion
- handle fence_server_sync for unsubmitted fences by
  relying on the improved cs_add_fence_dependency
- only implement asynchronous flushes on amdgpu

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
6 files changed