drm/nouveau: create real execution engine for software object class

Just a cleanup more or less, and to remove the need for special handling of
software objects.

This removes a heap of documentation on dma/graph object formats.  The info
is very out of date with our current understanding, and is far better
documented in rnndb in envytools git.

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