stash: move perf_oa_event_fd under brw_context->perfmon
stash: i965: expose B0 gpu clock counter for debugging
stash: i965: differentiate oa_users / open_oa_monitors state

Previously we would disable the OA counters in brw_end_perf_monitor()
via an LRI command, but now that we disable counter synchronously via an
ioctl we need to wait until we know that our last MI_RPC command has
been handled.