vkr: add VKR_RENDERER_ASYNC_FENCE_CB
This MR adds internal support for VKR_RENDERER_ASYNC_FENCE_CB
to vkr. The flag can be set when VIRGL_RENDERER_ASYNC_FENCE_CB
from !534 (merged) is set.
My hope is to merge this MR before !534 (merged), such that !534 (merged) can be more complete. Thoughts?