cpufreq: intel_pstate: Enable HWP without EPP if DEC is enabled

So far, HWP has never been enabled without EPP (Energy-Performance
Preference) interface support, since the lack of the latter indicates an
incomplete implementation of HWP, which was the case on early development
vehicle platforms.  However, HWP can be expected to work if DEC (Dynamic
Efficiency Control) is enabled as indicated by setting bit 27 in
MSR_IA32_POWER_CTL (DEC enable bit).

Accordingly, allow HWP to be enabled if the EPP interface is not
supported so long as DEC is enabled in the processor.

Still, the EPP control sysfs interface is useless when EPP is not
supported, so do not expose it in that case.

Link: https://lore.kernel.org/linux-pm/20250904000608.260817-2-srinivas.pandruvada@linux.intel.com/


Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Co-developed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>