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>

src/gallium/docs/source/context.rst

@@ -463,6 +463,16 @@ Flushing
 
 ``flush``
 
+PIPE_FLUSH_END_OF_FRAME: Whether the flush marks the end of frame.
+
+PIPE_FLUSH_DEFERRED: It is not required to flush right away, but it is required
+to return a valid fence. The behavior of fence_finish or any other call isn't
+changed. The only side effect can be that fence_finish will wait a little
+longer. No guidance is given as to how drivers should implement fence_finish
+with deferred flushes. If some drivers can't do deferred flushes safely, they
+should just ignore the flag.
+
+
 
 ``flush_resource``