sched: fix energy efficient return cpu

With recent upmerge commit 'dc6f7513("ANDROID: sched/fair:
return idle CPU immediately for prefer_idle")', the
find_energy_efficient_cpu ignores energy evaluated cpu.

Fix it by updating energy_cpu with energy evaluated cpu,
while at it, if efficient cpu not found, do not return -1 instead
return prev_cpu.

Change-Id: Icf269416b01a8cbade688af3e53ee59a664449a9
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>