sched: EAS/WALT: take into account of waking task's load

WALT's function cpu_util(cpu) reports CPU's load without taking into
account of waking task's load.  Thus currently cpu_overutilized()
underestimates load on the previous CPU of waking task.

Take into account of task's load to determine whether previous CPU is
overutilzed to bail out early without running energy_diff() which is
expensive.

Change-Id: I30f146984a880ad2cc1b8a4ce35bd239a8c9a607
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
(minor rebase conflicts)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>