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: eeaf06ac1a ("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>

drivers/gpu/drm/nouveau/nouveau_svm.c

@@ -315,6 +315,10 @@ nouveau_svmm_init(struct drm_device *dev, void *data,
 	struct drm_nouveau_svm_init *args = data;
 	int ret;
 
+	/* We need to fail if svm is disabled */
+	if (!cli->drm->svm)
+		return -ENOSYS;
+
 	/* Allocate tracking for SVM-enabled VMM. */
 	if (!(svmm = kzalloc(sizeof(*svmm), GFP_KERNEL)))
 		return -ENOMEM;