intel_pstate: Clarify average performance computation

The core_pct_busy field of struct sample actually contains the
average performace during the last sampling period (in percent)
and not the utilization of the core as suggested by its name
which is confusing.

For this reason, change the name of that field to core_avg_perf
and rename the function that computes its value accordingly.

Also notice that storing this value as percentage requires a costly
integer multiplication to be carried out in a hot path, so instead
store it as an "extended fixed point" value with more fraction bits
and update the code using it accordingly (it is better to change the
name of the field along with its meaning in one go than to make those
two changes separately, as that would likely lead to more
confusion).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>