gallium: add PIPE_FLUSH_DEFERRED

gallium: add PIPE_FLUSH_DEFERRED

There are 2 uses:
- Asynchronous flushing for multithreaded drivers.
- Return a fence without flushing (mid-command-buffer fence). The driver
  can defer flushing until fence_finish is called.

This is required to make Bioshock Infinite faster, which creates
1000 fences (flushes) per frame.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>