cpufreq: schedutil: Add util to struct sg_cpu

Instead of passing util and max between functions while computing the
utilization and capacity, store the former in struct sg_cpu (along
with the latter and bw_dl).

This will allow the current utilization value to be compared with the
one obtained previously (which is requisite for some code changes to
follow this one), but also it causes the code to look slightly more
consistent and cleaner.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>