drm/nouveau/svm: fail NOUVEAU_SVM_INIT ioctl on unsupported devices

Fixes a crash when trying to create a channel on e.g. Turing GPUs when
NOUVEAU_SVM_INIT was called before.

Fixes: eeaf06ac

 ("drm/nouveau/svm: initial support for shared virtual memory")
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>