sched: fix energy diff calculation when sync = 1

Make energy_diff() aware of sync flag and waker CPU in order to discount
energy cost of placing wakee task on the waker's CPU.

Change-Id: Ic9fa136167550307e2920fb1ea5ea340fe1b782d
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>