drm/nouveau/disp: add dp sst config method

This is presently unused on HW, we read a bunch of regs and calculate
the watermark during the second supervisor interrupt.

I don't want to change this yet as I need to re-remember how older HW
works exactly, but RM wants this info via RPC.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Danilo Krummrich <me@dakr.org>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230919220442.202488-35-lyude@redhat.com