cpufreq: intel_pstate: Eliminate intel_pstate_get_base_pstate()

There is only one caller of intel_pstate_get_base_pstate() and it is
more straightforward to carry out the computation directly in the
caller, so do that and drop intel_pstate_get_base_pstate().

No intentional changes of behavior.

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