swr: Limit memory held by defer deleted resources.

swr: Limit memory held by defer deleted resources.

This patch limits the number of items on the fence work queue (the
deferred deletion list) by submitting a sync fence when the queue size
exceeds a threshold.  This initiates deferred deletion of all resources
on the list and decreases the total amount of memory held waiting for
"deferred deletion".

This resolves  bug 101467 filed against swr for the piglit
streaming-texture-leak test.  For those running on smaller memory
(16GB?) systems, this will prevent oom-killer.

Thus far, we have not seen any real world applications that exhibit
behavior like the streaming-texture-leak test; as any form of pipeline
flush will trigger the defer queue and properly free any retained
allocations.  But, this addresses those as well.

Cc: "17.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>