drm: rename null fence to stub fence in syncobj v2

moved to front of file.
stub fence will be used by timeline syncobj as well.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/246539/