[PanelView] Check whether we're X11 before calling QX11Info::connection()

QX11Info::connection() looks up the connection in the QPA.
The Wayland QPA happens to have a property with the same name,
so it happily reinterpret_casts a wl_connection_t* to xcb_connection_t*
and disaster ensues.

Differential Revision: https://phabricator.kde.org/D1718