drm/nouveau/fifo: turn all fifo modules into engine modules

Been tested on each major revision that's relevant here, but I'm sure there
are still bugs waiting to be ironed out.

This is a *very* invasive change.

There's a couple of pieces left that I don't like much (eg. other engines
using fifo_priv for the channel count), but that's an artefact of there
being a master channel list still.  This is changing, slowly.

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