drm/nouveau: Replace ttm_bo_reference with ttm_bo_get

The function ttm_bo_get acquires a reference on a TTM buffer object. The
function's name is more aligned to the Linux kernel convention of naming
ref-counting function _get and _put.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/nouveau/nouveau_bo.h

@@ -61,7 +61,12 @@ nouveau_bo_ref(struct nouveau_bo *ref, struct nouveau_bo **pnvbo)
 		return -EINVAL;
 	prev = *pnvbo;
 
-	*pnvbo = ref ? nouveau_bo(ttm_bo_reference(&ref->bo)) : NULL;
+	if (ref) {
+		ttm_bo_get(&ref->bo);
+		*pnvbo = nouveau_bo(&ref->bo);
+	} else {
+		*pnvbo = NULL;
+	}
 	if (prev) {
 		struct ttm_buffer_object *bo = &prev->bo;