drm/nouveau/sec2: unload RTOS before tearing down WPR

Reset regs won't be available on Ampere while SEC2 RTOS is running, and
we're apparently supposed to be doing this on earlier GPUs too.

v2:
- fixed some excessive indentation

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