cpufreq: handle calls to ->target_index() in separate routine

Handling calls to ->target_index() has got complex over time and might become
more complex. So, its better to take target_index() bits out in another routine
__target_index() for better code readability. Shouldn't have any functional
impact.

Tested-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>