sched/fair: Force gold cpus to do idle lb when silver has big tasks

We often see traces that gold cpus goes to idle even when silver
has big tasks. This is because avg_idle checks does not allow
the gold cpus to enter idle balance. This is NOT okay when
silver CPUs have big tasks.

Change-Id: Ice00d43c70be105b5bc3ad09caa6047aa98d7402
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Runmin Wang <runminw@codeaurora.org>