drm/nouveau/core: convert event handler apis to split create/enable semantics

This is a necessary step towards being able to work with the insane locking
requirements of the DRM core's vblank routines, and a nice cleanup as a
side-effect.

This is similar in spirit to the interfaces that Peter Hurley arrived at
with his nouveau_event rcu conversion series.

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