PM / sleep: Move platform suspend operations to separate functions

After the introduction of freeze_ops it makes more sense to move
all of the platform suspend operations to separate functions that
each will do all of the necessary checks and choose the right
callback to execute istead of doing all that in the core code
which makes it generally harder to follow.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>