PM: runtime: Relocate rpm_callback() right after __rpm_callback()

Because rpm_callback() is a wrapper around __rpm_callback(), and the
only caller of it after the change eliminating an invocation of it
from rpm_idle(), move the former next to the latter to make the code
a bit easier to follow.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>