PM / Runtime: Reduce code duplication in core helper functions

Reduce code duplication in rpm_idle(), rpm_suspend() and rpm_resume()
by using local pointers to store callback addresses and moving some
duplicated code into a separate function.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>