cpufreq: intel_pstate: Clean up frequency computations

Notice that some computations related to frequency in intel_pstate
can be simplified if (a) intel_pstate_get_hwp_max() updates the
relevant members of struct cpudata by itself and (b) the "turbo
disabled" check is moved from it to its callers, so modify the code
accordingly and while at it rename intel_pstate_get_hwp_max() to
intel_pstate_get_hwp_cap() which better reflects its purpose and
provide a simplified variat of it, __intel_pstate_get_hwp_cap(),
suitable for the initialization path.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>