Input: mousedev - fix sudden warps with touchpads

Pete Zaitcev reports that with his touchpad, if he lifts the finger
and places it elsewhere, the pointer sometimes warps dramatically.
This happens because we don't store coordinates unless we detect a
touch so sometimes we have stale coordinates in queue (from where
the finger left the pad) and averaging makes cursor to jump across
the screen. The solution is to always store the latest coordinates.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>