Fix invalidation of cached x stacking order in wayland only mode
Currently, if kwin/wayland runs without xwayland, the order in which windows are painted doesn't actually reflect the true stacking order. If the stacking order has been changed, we need to invalidate the cached x stacking order. But it's done only when RootInfo is present. If Xwayland doesn't run, RootInfo is not available and thus window raising is completely broken. With this change, the x stacking order will be invalidated every time some window has been raised, no matter what mode kwin operates in.