sched: revise boost logic when boost_type is SCHED_BOOST_ON_BIG

At present HMP scheduler boost tends to pack tasks by taking into
account of power cost and cstate.  This is suboptimal to performance
as it can lead preemption and higher latency.

Revise logic to prefer the least loaded CPU among the big cluster CPUs
when boost type is SCHED_BOOST_ON_BIG.  New logic still honor the
behaviour that scheduler can place tasks on the little CPUs when the
big CPUs are all overcommitted.

Also, it's found that need_idle with boost can easily return previous
CPU when there is no idle CPU found.  Fix this issue by making
need_idle flag to take precedence over sched_boost.

CRs-fixed: 1074879
Change-Id: I470bcd0588e038b4a540d337fe6a412f2fa74920
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>