sched: add preference for prev_cpu in HMP task placement

At present the HMP task placement algorithm scans CPUs in numerical
order and if two identical options are found, the first one
encountered is chosen, even if it is different from the task's
previous CPU.

Add a bias towards the task's previous CPU in such situations. Any
time two or more CPUs are considered equivalent (load, C-state, power
cost), if one of them is the task's previous CPU, bias towards that
CPU. The algorithm is otherwise unchanged.

CRs-Fixed: 772033
Change-Id: I511f5b929c2bfa6fdea9e7433893c27b29ed8026
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>