sched: optimize best_small_task_cpu() loop

Optimize function best_small_task_cpu() with followings:

- Return immediately once idle CPU which isn't a low power mode.
- Reduce calls of scale_load_to_cpu() when energy aware scheduler is off.
- Clear search_cpus later.

CRs-fixed: 849655
Change-Id: I15ff4856ac476797547ec5e023b2897cbae935d8
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>