cpufreq: don't leave stale policy pointer in cdbs->cur_policy

cpufreq: don't leave stale policy pointer in cdbs->cur_policy

Clear ->cur_policy when stopping a governor, or the ->cur_policy
pointer may be stale on systems with have_governor_per_policy when a
new policy is allocated due to CPU hotplug offline/online.

[rjw: Changelog]
Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jacob Shin <jacob.shin@amd.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>