Do not call damage or commit before calling attach

We need to check that a buffer has been attached first before calling
damage and commit on the wayland client side.  So now we do in
QWaylandWindow.

Change-Id: I902dc5b479a2af10009de6acb42d6b455a6e4cb5
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>