cpufreq: intel_pstate: Use HWP capabilities in intel_cpufreq_adjust_perf()

If turbo P-states cannot be used, either due to the configuration of
the processor, or because intel_pstate is not allowed to used them,
the maximum available P-state with HWP enabled corresponds to the
HWP_CAP.GUARANTEED value which is not static.  It can be adjusted by
an out-of-band agent or during an Intel Speed Select performance
level change, so long as it remains less than or equal to
HWP_CAP.MAX.

However, if turbo P-states cannot be used, intel_cpufreq_adjust_perf()
always uses pstate.max_pstate (set during the initialization of the
driver only) as the maximum available P-state, so it may miss a change
of the HWP_CAP.GUARANTEED value.

Prevent that from happening by modifyig intel_cpufreq_adjust_perf()
to always read the "guaranteed" and "maximum turbo" performance
levels from the cached HWP_CAP value.

Fixes: a365ab6b

 ("cpufreq: intel_pstate: Implement the ->adjust_perf() callback")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>