drm/nouveau/flcn: rework falcon reset

Mostly preparation to fit in Ampere changes, but should result in reset
sequences a lot closer to RM's, and perhaps help out with the issues we
sometimes see reported in this area.

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