This doesn't attach a buffer to the surface. This is needed for the
next commit, where we have a test case with a surface that doesn't
have a buffer attached.
Signed-off-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>