radeonsi: prevent a deadlock in util_queue_add_job with too many GL contexts

radeonsi: prevent a deadlock in util_queue_add_job with too many GL contexts

If the queue is full, util_queue_add_job will wait while bo_fence_lock is
held.

It pb_slab wants to reuse a buffer, it will lock the pb_slab mutex and
try to check BO fence busyness, but it has to wait for bo_fence_lock to get
released. Both bo_fence_lock and pb_slab mutex are locked now.

When the CS thread unreferences and releases a suballocated buffer,
it will try to lock the pb_slab mutex and has to wait. The CS thread
can't finish its job in order to free a queue slot and unblock
util_queue_add_job ==> deadlock.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>