cpufreq: interactive: Use generic get_cpu_idle_time() from cpufreq.c

Now that a generic version of get_cpu_idle_time() is available, use that for the interactive governor.

[toddpoynor@google.com: commit text changes]
Change-Id: Ia38b57085aac99ec3d415fe44471d5dfde519c2c
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>