cpufreq: intel_pstate: Eliminate some code duplication

cpufreq: intel_pstate: Eliminate some code duplication

To eliminate some code duplication from the intel_pstate driver,
move the core_get_val() function body to a new function called
get_perf_ctl_val() and make both core_get_val() and atom_get_val()
invoke it to carry out the same computation.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://patch.msgid.link/2829273.mvXUDI8C0e@rafael.j.wysocki