sched_ext/scx_qmap: Pick idle CPU for direct dispatch on !wakeup enqueues

Because there was no way to directly dispatch to the local DSQ of a remote
CPU from ops.enqueue(), scx_qmap skipped looking for an idle CPU on !wakeup
enqueues. This restriction was removed and sched_ext now allows
SCX_DSQ_LOCAL_ON verdicts for direct dispatches.

Factor out pick_direct_dispatch_cpu() from ops.select_cpu() and use it to
direct dispatch from ops.enqueue() on !wakeup enqueues.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>
Cc: Changwoo Min <changwoo@igalia.com>
Cc: Andrea Righi <righi.andrea@gmail.com>