anv: Stop using cs_prog_data->threads

anv: Stop using cs_prog_data->threads

Move the calculation to helper functions -- similar to what GL already
needs to do.

This is a preparation for dropping this field since this value is
expected to be calculated by the drivers now for variable group size
case.  And also the field would get in the way of brw_compile_cs
producing multiple SIMD variants (like FS).

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4504>