sched/fair: ensure utilization signals are synchronized before use

sched/fair: ensure utilization signals are synchronized before use

wake_cap performs task and cpu utilization synchronization which is
what allows us to subtract current task util from prev_cpu util and
have a sensible number to work with.

It looks as though if wake_wide returns 0, we could potentially not
execute wake_cap, which would result in unsynced signals we then use
for energy calculations.

This is not necessarily an issue we've seen in traces, but it looks
as though it should be changed.

Change-Id: Ic54a3cba2a10d946ea20113a04371dea04115e82
Signed-off-by: Chris Redpath <chris.redpath@arm.com>