sched/fair: Refactor cpu_util_without()

Except the `task has no contribution or is new` condition at the
beginning of cpu_util_without() a cpu_util_next(..., dst_cpu = -1)
call can replace the rest of the function.

The UTIL_EST related check for the special handling of the race for an
enqueued of running WF_EXEC task between select_task_rq_fair() and
load-balance's detach_task() has to be moved from cpu_util_without()
to cpu_util_next(). It was initially introduced by commit c469933e
("sched/fair: Fix cpu_util_wake() for 'execl' type workloads").

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>