llvmpipe: compute shaders work better with all the threads.
I got to benchmarking some vulkan compute benchmark and wondered why my CPUs weren't being saturated, helps if you actually wake up all the threads in the threadpool. Fixes: 1b24e3ba (llvmpipe: add compute threadpool + mutex) Reviewed-by:Roland Scheidegger <sroland@vmware.com> Part-of: <mesa/mesa!5138> (cherry picked from commit 22554e1f)