drm/nouveau/intr: add nvkm_subdev_intr() compatibility

It's quite a lot of tedious and error-prone work to switch over all the
subdevs at once, so allow an nvkm_intr to request new-style handlers to
be created that wrap the existing interfaces.

This will allow a more gradual transition.

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