Fix generation of proximity-in/out events.

Invoking xf86PostProximityEvent with no valuators does nothing, so we
have to provide a valid valuator set to the call.

https://bugs.freedesktop.org/show_bug.cgi?id=104562



[whot: slight change from 104562
abs_vals is reset on every frame but old_vals is kept around to remember the
last value of the valuator (and is updated to the current values before
processing the frame). We expect the prox in to have x/y, so let's just
unconditionally use old_vals for proximity events.]

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