drm/dp_mst: Remove all evil duplicate state pointers

There's no reason to track the atomic state three times. Unfortunately,
this is currently what we're doing, and even worse is that there is only
one actually correct state pointer: the one in mst_state->base.state.
mgr->state never seems to be used, along with the one in
mst_state->state.

This confused me for over 4 hours until I realized there was no magic
behind these pointers. So, let's save everyone else from the trouble.

Signed-off-by: Lyude Paul <lyude@redhat.com&gt;.>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>