sched: Relocated cpu_util() and change return type

Move cpu_util() to an earlier position in fair.c and change return
type to unsigned long as negative usage doesn't make much sense. All
other load and capacity related functions use unsigned long including
the caller of cpu_util().

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>