drm/nouveau/core: Add priv field for event handlers

Provide private field for event handlers exclusive use.
Convert nouveau_fence_wait_uevent() and
nouveau_fence_wait_uevent_handler(); drop struct nouveau_fence_uevent.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>