drm/nouveau/device: convert user class to new-style nvkm_object

drivers/gpu/drm/nouveau/include/nvkm/core/device.h

drivers/gpu/drm/nouveau/nvkm/core/client.c

drivers/gpu/drm/nouveau/nvkm/engine/device/base.c

drivers/gpu/drm/nouveau/nvkm/engine/device/user.c

drivers/gpu/drm/nouveau/nvkm/engine/disp/nv50.c

drivers/gpu/drm/nouveau/nvkm/engine/dmaobj/base.c

drivers/gpu/drm/nouveau/nvkm/engine/pm/base.c