sched/fair: Skip energy_diff() for need_idle tasks

The need_idle eligible tasks are placed on the least loaded
CPUs including idle CPUs. The energy_diff() can overturn this
decision if utilizing this idle CPU results in higher power.
Since need_idle eligible tasks scheduling latency is critical
for performance, skip energy_diff() for such tasks.

Change-Id: Ic72c0ed9461a5f91a2f0de055765fd9b2457cdde
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>