drm/nouveau/TBDdevinit: don't fail when PMU/PRE_OS is missing from VBIOS

This Falcon application doesn't appear to be present on some newer
systems, so let's not fail init if we can't find it.

TBD: is there a way to determine whether it *should* be there?

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