cpufreq: Reuse cpufreq_driver_resolve_freq() in __cpufreq_driver_target()

__cpufreq_driver_target() open codes cpufreq_driver_resolve_freq(), lets
make the former reuse the later.

Separate out __resolve_freq() to accept relation as well as an argument
and use it at both the locations.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>