sched/fair: Don't miss out an idle CPU when placement boost is active

When placement boost is active, the lowest capacity CPUs are skipped.
The CPU with the highest spare capacity in the higher capacity
clusters is selected. The highest capacity cluster can slightly have
higher spare capacity compared to an idle CPU of the higher capacity
cluster. This results in placing a task on an active CPU. Instead look
for an idle CPU in the higher capacity clusters. If such CPU is not
available, the task is placed on the CPU with highest spare capacity.

Change-Id: I45efd9dfe96480b78e8a5adc119306932eac7b11
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>