PM: sleep: Call dpm_async_fn() directly in each suspend phase

Simplify the system-wide suspend of devices by invoking dpm_async_fn()
directly from the main loop in each suspend phase instead of using an
additional wrapper function for running it.

No intentional functional impact.

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