thermal: core: Rearrange PM notification code

Move the code run for each thermal zone by the thermal PM notify
handler to separate functions.

This will help to make some subsequent changes look somewhat more
straightforward, among other things.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2299090.iZASKD2KPV@rjwysocki.net


Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>