nohz: fix idle and iowait stats discrepancy

There is a difference in idle and iowait accounting between
cpustat(periodic ticks) and tick_cpu_sched(nohz). Currently
when CPU is not online, cpustat based values are used. When
the CPU comes online, we start returning values based on
tick_cpu_sched. This breaks the monotonic nature of these
stats.

When nohz is enabled, don't fallback to cpustat for idle and
iowait stats instead use the accumulated values in tick_cpu_sched().

Change-Id: I768e80224012a6907520e2f6a301c3ebb29ed27d
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>