tablet: fix double proximity out on slow proximity out pens

Where the proximity out event is delayed by the kernel, libinput would cause
an extra proxmity in-out after the forced proximity out event.

Event sequence is basically (k: kernel, l: libinput)

k: tablet axis events
l: tablet axis events
k: nothing for $proximity timer milliseconds
l: tablet proximity out
k: proximity out event
l: proximity in event
l: proximity out event

Fixes #306

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