drm/nouveau/disp: add supervisor mutex

Will be used to protect NVIF_CLASS_OUTP method calls from racing with
in-progress supervisor handling.

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