WIP: drm/amdgpu/dm/todo: Add some suggestions

I've been busy enough that I haven't really had the time to address this
but: there's a number of things that amdgpu really, really should be doing
maintenance wise that don't seem to be happening on their own. Some of
these were spurred by having to fix a regression with MST just now, but
most of these have been in my head for a while now.

Let's actually put them into the TODO so we have something more concrete to
point people at for what we'd like to see in this driver in the future.

Signed-off-by: Lyude Paul <lyude@redhat.com>