gallium/u_queue: guard fence->signalled checks with fence->mutex

gallium/u_queue: guard fence->signalled checks with fence->mutex

I have seen a hang during application shutdown that could be explained by the
following race condition which this patch fixes:

1. Worker thread enters util_queue_fence_signal, sets fence->signalled = true.
2. Main thread calls util_queue_job_wait, which returns immediately.
3. Main thread deletes the job and fence structures, leaving garbage behind.
4. Worker thread calls pipe_condvar_broadcast, which gets stuck forever because
   it is accessing garbage.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>