cpufreq / intel_pstate: Optimize intel_pstate_set_policy

This function is called quite often from other subsystems.
Removed unused call to intel_pstate_get_min_max().
Also when "policy->policy == CPUFREQ_POLICY_PERFORMANCE", then
no need to do calculations as the limits will be forced anyway.
Also corrected filename in the header.

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