wip: drm/dp_mst: Add ->pre_enable/->post_disable() hooks

Some drivers, nouveau in particular, need to perform some hardware setup
before enabling the MST topology. Ideally, we want this setup to always
be done whenever topologies are enabled or disabled, even if it's not
because the driver choose to change the topology state. So, add some
hooks that get called under the main MST topology manager's lock and
implement said hooks in nouveau.

TODO: test this works