PM: runtime: Introduce __rpm_get_driver_callback()

Add a special function for computing the address of the runtime PM
callback given by an offset relative to the start of the device
driver's struct dev_pm_ops and use it to obtain the driver callback
in __rpm_get_callback().

Also put the shared part of the callback address computation into a
separate helper function to avoid code duplication and explicit
pointer type casts.

The new __rpm_get_driver_callback() will be used subsequently for
implementing callback lookup in pm_runtime_force_suspend/resume().

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/2054356.usQuhbGJ8B@rjwysocki.net