ivi-layout.c: remove unneeded casts

Also a couple of whitespace fixes.
ivi-layout.c: reflow long lines
ivi-layout.c: simplify add_link_to_surface()

There is a way to make this function even simpler.

If you guarantee, that it is always legal to call wl_list_remove() on the
link, this function could literally be just:

wl_list_remove(&link_layer->link_to_layer);
wl_list_insert(&ivilayer->link_to_surface, &link_layer->link_to_layer);

You would go from an O(n) operation to O(1) operation!