cpufreq: call cpufreq_driver->get() after calling ->init()

Almost all drivers set policy->cur with current CPU frequency in their ->init()
part. This can be done for all of them at core level and so they wouldn't need
to do it.

This patch adds supporting code in cpufreq core for calling get() after we have
called init() for a policy.

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