sched/walt: Fix nr_eligible_big_tasks() for tri-cluster systems

nr_eligible_big_tasks() should return the number BIG tasks running
on the lowest capacity cluster. For higher capacity clusters, it
should return simply the number of runnable tasks. This assumption
is currently broken for tri-cluster systems. Fix it.

Change-Id: Ia451e965bf9c37ba181225c4beefbd77e6d1cead
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>