drm/nouveau/nvkm: rip out event uapi

Userspace never ended up using this to be clever about dealing with
channel death, and it won't be, not like this anyway.

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