drm/nouveau/nvkm: add locking to subdev/engine init paths

This wasn't really needed before; the main place this could race is with
channel recovery, but (through potentially fragile means) shouldn't have
been possible.

However, a number of upcoming patches benefit from having better control
over subdev init, necessitating some improvements here.

- allows subdev/engine oneinit() without init() (host/fifo patches)
- merges engine use locking/tracking into subdev, and extends it to fix
  some issues that will arise with future usage patterns (acr patches)

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