vk: Compute CS exec mask and thread width max in pipeline

vk: Compute CS exec mask and thread width max in pipeline

We compute the right mask and thread width max parameters as part of
pipeline creation and set them accordingly at vkCmdDispatch() and
vkCmdDispatchIndirect() time. These parameters depend only on the local
group size and the dispatch width of the program so we can figure this
out at pipeline create time.