async: Split async_schedule_node_domain()

In preparation for subsequent changes, split async_schedule_node_domain()
in two pieces so as to allow the bottom part of it to be called from a
somewhat different code path.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Tested-by: Youngmin Nam <youngmin.nam@samsung.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>