drm/nouveau/secboot: check that WPR region is properly set

The ACR firmware may return no error but fail nonetheless. Such cases
can be detected by verifying that the WPR region has been properly set
in FB. If this is not the case, this is an error, but the unload
firmware should still not be run.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>