cpufreq: intel_pstate: Introduce pid_in_use()

Add a new function pid_in_use() to return the information on whether
or not the PID-based P-state selection algorithm is in use.

That allows a couple of complicated conditions in the code to be
reduced to simple checks against the new function's return value.

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