intel/compiler: Replace cs_prog_data->push.total with a helper

intel/compiler: Replace cs_prog_data->push.total with a helper

The push.total field had three values but only one was directly
used (size).  Replace it with a helper function that explicitly takes
the cs_prog_data and the number of threads -- and use that in the
drivers.

This is a preparation for ARB_compute_variable_group_size where the
number of threads (hence the total size for push constants) is not
defined at compile time (not cs_prog_data->threads).

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