[server] Don't emit unmapped if the Surface wasn't mapped

Summary:
If a Surface doesn't have a buffer attached and a null buffer gets
attached the buffer state doesn't really change. Thus neither the
unmapped signal nor the damaged signal should not be emitted.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1261