cpufreq: intel_pstate: Consolidate policy verification

There is still some code duplication between intel_pstate_verify_policy()
and intel_cpufreq_verify_policy(), so avoid it by moving the common
code into a separate function and calling it from both these places.

No intentional functional impact.

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