main/postmarketos-ui-{mate,xfce4}: disable lightdm service on upgrade

Largely fixes https://gitlab.com/postmarketOS/pmaports/-/merge_requests/3887#note_1285444312.

However seeing these are not mobile UI's but rather desktop ones, the switch to tinydm was really a downgrade as now these UI's can only autologin rather than offering UI selection and user switching like it's expected from a desktop. If the only goal of moving to tinydm was working around the regression that came about when pbsplash was merged, then I think we should revert the move to tinydm and properly fix pbsplash instead.