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.


(cherry picked from commit 75cad57c)