drm/nouveau/priv: punt messages to debug level

Ideally we'd be able to keep these at a more obvious error level, as
they're a good indication of us doing something wrong.

However, NVIDIA's FECS/GPCCS firmware touches registers that trigger
priv ring faults, and we can't do anything to fix that ourselves due
to the need for them to be signed by NVIDIA.

This issue was reported a while back, but hasn't been fixed, so, for
now we will hide the messages to prevent spamming Optimus users with
messages whenever the NVIDIA GPU is powered off and on again.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>