wayland: Don't overwrite surface offsets

The intention when the offset request was added to protocol was
that the attach request in a new enough protocol version should
require dx/dy to be zero, but ignore them otherwise.

The current code checks for 0, but then overwrites the existing
dx/dy with it, which renders an earlier wl_surface_offset() call
ineffective.

Fixes: #2622
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2843>


(cherry picked from commit ea373cb0)