sched: WALT: don't bias towards prev CPU when ediff == 0

The target_cpu can be often better choice than previous CPU considering
CPU load.  Always place task on target_cpu rather than previous CPU
when energy cost difference is same.

Change-Id: Id47b5a0c5dbbbbb0d7c43e7dd5f2d958683bc1a7
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>