drm/nv04/disp: hide all the cruft away in its own little hole

It'd be pretty awesome if someone would care enough to port this all
properly to a class interface, perhaps submitting a command stream to
the core via a sw object on PFIFO (emulating how EVO works basically,
and also what nvidia have done forever..)..

But, this seems unlikely given how old this hardware is now, so, lets
just hide it away.

There's a heap of other bits and pieces laying around that are still
tangled. I'll (re)move them in pieces.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>