gallium/u_queue: add optional cleanup callback

gallium/u_queue: add optional cleanup callback

Adds a second optional cleanup callback, called after the fence is
signaled.  This is needed if, for example, the queue has the last
reference to the object that embeds the util_queue_fence.  In this
case we cannot drop the ref in the main callback, since that would
result in the fence being destroyed before it is signaled.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>