Ignore x/y axis changes on proximity out

Wacom tablets send a farewell reset to 0 on all axes when the tool goes out of
proximity. Ignore those so we can rely on our various valuator masks always
having the correct coordinates.

Possible false positive if you manage to go out of proximity right above the
zero datum but meh.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>