ANDROID: sched/fair: Bypass energy computation for prefer_idle tasks

If the only pre-selected candidate CPU in find_energy_efficient_cpu()
happens to be prev_cpu, there is not point in computing the system
energy since we have nothing to compare it against, so we currently bail
out early. The same logic can be extended when prefer_idle tasks are
routed in the energy-aware wake-up path: if the only candidate is idle
for a prefer_idle task, just select it no matter what the energy impact
is. That should help speeding-up wake-ups of prefer_idle tasks, at least
when find_best_target() is used for them.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Change-Id: Idd0e387e4a766061cc05d2584df3a31e4dabfd09