sched/fair: Help out higher capacity CPUs when they are overcommitted

This comprises of two parts:

If we have a task to schedule, we currently don't consider CPUs where
it will not fit even if they are idle. Instead we choose the previous
CPU which is sub-optimal for performance if an idle CPU is
present. This change introduces tracking of any idle CPUs irrespective
of whether the task fits on them or not. If we don't have a good place
to put the task, prefer the lowest power idle CPU.

The other part involves the load balancer which was unable to move
tasks despite the above mentioned task placement to balance out the
load. The reason is that the load balancer checks the big cluster's
group capacity and determines that it can take twice the amount of
workload as the little cluster. Hence the big cluster does not get
marked as busy. While this behavior is intended under heavily loaded
systems where we want to push more work towards the higher capacity
CPUs, it is sub optimal when we have idle CPUs. Add the ability to
differentiate between the two scenarios when marking a group as
busy. If load_balance is called from a CPU_NOT_IDLE environment use
the the group capacity to determine whether the group is busy or
not. For everything else use number of CPUs in the group.

Change-Id: I4e8290639ad1602541a44a80ba4b2804068cac0f
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>