cpufreq: intel_pstate: Do not expose PID parameters in passive mode

If intel_pstate works in the passive mode in which it acts as
a regular cpufreq driver and collaborates with generic cpufreq
governors, the PID parameters are not used, so do not expose
them via debugfs in that case.

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