cpufreq: intel_pstate: Rearrange the storing of new EPP values

Move the locking away from intel_pstate_set_energy_pref_index()
into its only caller and drop the (now redundant) return_pref label
from it.

Also move the "raw" EPP value check into the caller of that function,
so as to do it before acquiring the mutex, and reduce code duplication
related to the "raw" EPP values processing somewhat.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>