drm/nouveau/nvkm: add macros for subdev layout

Rather than having to add new engines / engine instances to multiple places,
define everything in include/nvkm/core/layout.h and use macros to generate
the required plumbing.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>