cpufreq: intel_pstate: Add locking around HWP requests

To avoid race conditions from multiple threads, increase the scope
of intel_pstate_limits_lock to include HWP requests also.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>