sched/schedutil: Use sched_set_dl()

While schedutil is part of the scheduler, but its use of sched_setattr()
will prevent it us from making it static, hence properly prevent all
other in-kernel users from directly accessing it.

The runtime value has changed, but according to the comment it was fake
value to deal with priority inheritance.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>