display: Set compositor selection earlier on XWayland
When the X11 display is actually XWayland there's no point to delay the compositor selection, given that mutter itself is the compositor and doing this may cause the first X11 client that starts not to receive the right information (and in some cases misbehave).
Since some toolkits are not handling the compositor selection changes properly at later times, let's make their life easier by just initializing the selection as early as the other X11 properties, given that in this case there's nothing to replace.
Fixes: #2472 (closed)