drm/i915: Add fb format modifier support

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.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>