drm/nouveau/gr/tu102-: prepare for GSP-RM

- (temporarily) disable if GSP-RM detected, will be added later
- make init() optional

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918202149.4343-27-skeggsb@gmail.com