drm/nouveau/svm: check for SVM initialized before migrating

When migrating system memory to GPU memory, check that SVM has been
enabled. Even though most errors can be ignored since migration is
a performance optimization, return an error because this is a violation
of the API.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>