drm/nouveau: working towards a common way to represent engines

There's lots of more-or-less independant engines present on NVIDIA GPUs
these days, and we generally want to perform the same operations on them.
Implementing new ones requires hooking into lots of different places,
the aim of this work is to make this simpler and cleaner.

NV84:NV98 PCRYPT moved over as a test.

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