cpufreq: intel_pstate: Avoid enabling HWP if EPP is not supported

Although there are processors supporting hardware-managed P-states
(HWP) without the energy-performance preference (EPP) feature, they
are not expected to be run with HWP enabled (the BIOS should disable
HWP on those systems).  Missing EPP support generally indicates an
incomplete HWP implementation and so it is better to avoid using
HWP on those systems in production.

However, intel_pstate currently enables HWP on such systems, which
is questionable, so prevent it from doing that by making it check
EPP support before enabling HWP and avoid enabling it if EPP is not
supported by the processor at hand.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>