ir3: Create at least one thread for async shader compile

ir3: Create at least one thread for async shader compile

On a single core CPU system we end with num_threads being 0 and we do
_NOT_ create any async compile threads in util_queue_init(..).

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17124>