util/u_queue: rework UTIL_QUEUE_INIT_SCALE_THREADS to scale faster

util/u_queue: rework UTIL_QUEUE_INIT_SCALE_THREADS to scale faster

The original code waiting for the queue to be full before adding more
threads. This makes the thread count grow slowly, especially if the
queue also uses UTIL_QUEUE_INIT_RESIZE_IF_FULL.

This commit changes this behavior: now a new thread is spawned if we're
adding a job to a non-empty queue because this means that the existing
threads fail to process jobs faster than they're queued.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16273>