drm/nouveau/disp: add common channel class handling

Replaces a bunch of unnecessarily duplicated boilerplate in per-chipset
code with a simpler, common, implementation.

Channel "awaken" notify code is completely gone for now.  KMS has never
made use of it so far, and event notify handling is about to be changed
in general anyway.

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