sched: look for least busy and fallback CPU only when it's needed

Function best_small_task_cpu() has bias on mostly idle CPUs and shallow
cstate CPUs.  Thus chance of needing to find the least busy or the least
power cost fallback CPU is quite rare typically.  At present, however,
the function finds those two CPUs always unnecessarily for most of time.

Optimize the function by amending it to look for the least busy CPU and
the least power cost fallback CPU only when those are in need.  This change
is solely for optimization and doesn't make functional changes.

CRs-fixed: 849655
Change-Id: I5eca11436e85b448142a7a7644f422c71eb25e8e
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>