drm/nouveau: platform: Fix deferred probe

The error cleanup paths aren't quite correct and will crash upon
deferred probe.

Cc: stable@vger.kernel.org # v4.3+
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>