cpufreq: intel_pstate: Combine ->stop_cpu() and ->offline()

Combine the ->stop_cpu() and ->offline() callback routines for
intel_pstate in the active mode so as to avoid setting the
->stop_cpu callback pointer which is going to be dropped from
the framework.

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