gallium: extend pipe_context::flush for it to accept an END_OF_FRAME flag

gallium: extend pipe_context::flush for it to accept an END_OF_FRAME flag

Usage with pipe_context:
  pipe->flush(pipe, NULL, PIPE_FLUSH_END_OF_FRAME);

Usage with st_context_iface:
  st->flush(st, ST_FLUSH_END_OF_FRAME, NULL);

The flag is only a hint for drivers. Radeon will use it for buffer eviction
heuristics in the kernel (e.g. for queries like how many frames have passed
since a buffer was used).

The flag is currently only generated by st/dri on SwapBuffers.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>