Already in place for some functions, let's add it to most others.
The only function missing is miPointerSetPosition() which needs to
return the ScreenPtr and that one is unclear if we don't have a screen -
returning NULL will crash the caller(s) so let's wait for something to
trigger this bug before we try to fix it wrongly.
Related to #1782
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1771>