util/disk_cache: bump thread count assigned to disk cache queue

Since we set the UTIL_QUEUE_INIT_USE_MINIMUM_PRIORITY flag this should
have little impact on low core systems. However just about all modern
CPUs currently available that run Mesa have *at least* 4 cores. For
these CPUs allowing more threads can result in the queue being
processed faster and avoid excessive memory use due to a backlog of
cache entrys building up in the queue.

This change helps avoid a huge build up of cache entrys in the queue
due to using both the UTIL_QUEUE_INIT_USE_MINIMUM_PRIORITY and
UTIL_QUEUE_INIT_RESIZE_IF_FULL flags.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>