sched/fair: skip 6% energy check when best_energy_cpu is idle

Even though best_energy_cpu is idle and which would consume
same amount of enery as prev cpu, due to 6% check with prev cpu,
we end up chosing prev cpu over best_energy_cpu for the tasks.
This leads to increase in runnables. Choose best_energy_cpu when
it is idle and shallowest c-state over prev cpu in such scenarios
to avoid runnables.

Change-Id: Ib9eee263640bcd43bd80f8f777a8312585991bc5
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>