intel_pstate: enable HWP per CPU

HWP previously was only enabled at driver load time, on the boot
CPU, however, HWP must be enabled per package. Move the code to
enable HWP to the cpufreq driver init path so that it will be
called per CPU.

Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Tested-by: David Zhuang <david.zhuang@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>