drm/nouveau/gr/gf100: handle secure boot errors

Handle and propagate secure boot errors. Failure to do so results in
Nouveau incorrectly believing init has succeeded and a completely
black display during boot. If we propagate the error, GR init will fail
and the user will at least have a working display.

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