drm/virtio: implement prime mmap

Sync gem vm_node.start with ttm vm_node.start,
then we can just call drm_gem_prime_mmap().
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190227144441.6755-2-kraxel@redhat.com

drivers/gpu/drm/virtio/virtgpu_prime.c

@@ -56,7 +56,10 @@ void virtgpu_gem_prime_vunmap(struct drm_gem_object *obj, void *vaddr)
 }
 
 int virtgpu_gem_prime_mmap(struct drm_gem_object *obj,
-		       struct vm_area_struct *area)
+			   struct vm_area_struct *vma)
 {
-	return -ENODEV;
+	struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
+
+	bo->gem_base.vma_node.vm_node.start = bo->tbo.vma_node.vm_node.start;
+	return drm_gem_prime_mmap(obj, vma);
 }