cpufreq: interactive: Use sched_get_cpus_busy() to query busy time

sched_get_cpus_busy() provides a snapshot of all CPUs' busy time
information for the set of CPUs being queried. This avoids race
condition due to migration when CPU load is queried one by one.

Change-Id: I6afdfa74ff9f3ef616872df4e2c3bb04f6233c3f
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>