Currently we don't support anything but X tiled. And for an easier
transition it makes a lot of sense to just keep requiring that X tiled
is properly fenced.
Which means we need to do absolutely nothing in old code to support fb
modifiers, yay!
v2: Fix the Y tiling check, noticed by Tvrtko.
v3: Catch Y-tiled fb for legacy addfb again (Tvrtko) and explain why
we want X tiling to match in the comment.