winsys/amdgpu: add back multithreaded command submission

winsys/amdgpu: add back multithreaded command submission

Ported from the initial amdgpu winsys from the private AMD branch.

The thread creates the buffer list, submits IBs, and cleans up
the submission context, which can also destroy buffers.

3-5% reduction in CPU overhead is expected for apps submitting a lot
of IBs per frame. This is most visible with DMA IBs.

v2: use a semaphore instead of a busy loop in amdgpu_ws_queue_cs
    add another amdgpu_cs_sync_flush call into amdgpu_bo_map

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>