drm/nouveau/device: simplify subdev construction

Replaces the piece-by-piece (in response to NV_DEVICE ctor args) device
contruction with a once-off all-or-nothing approach, eliminating some
tricky refcounting issues.  The partial device init capability was only
required by some tools, and has been moved to probe time instead.

Temporarily removes a workaround for some boards where we need to fiddle
with AGP registers before executing the DEVINIT scripts.  A later commit
in this series reinstates it.

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