Check whether it's platform xcb before calling QX11Info::display()

Internally QX11Info::display() calls into QPA native interface and
tries to resolve the nativeResourceForIntegration for "display".
Unfortunately this key also exists in other QPA plugins, e.g. in
QtWayland.

Calling without a check on platform results on wayland qpa in
wl_display* being casted into Display*.

REVIEW: 124203

libkwineffects/kwinglobals.h

@@ -131,7 +131,7 @@ inline
 KWIN_EXPORT Display* display()
 {
     static Display *s_display = nullptr;
-    if (!s_display) {
+    if (!s_display && QX11Info::isPlatformX11()) {
         s_display = QX11Info::display();
     }
     return s_display;