i965: Start and stop OA counters as necessary.

i965: Start and stop OA counters as necessary.

We need to start OA at the beginning of each batch where monitors are
active.  OACONTROL isn't part of the hardware context, so to avoid
leaving counters enabled for other applications, we turn them off at the
end of the batch too.

We also need to start them at BeginPerfMonitor time (unless they've
already been started).  We stop them when the monitor last ends as well.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>