evdev: Flush motion events when the slot changes, not just after sync

evdev: Flush motion events when the slot changes, not just after sync

If two fingers are released almost simultaneously then evdev can send
the touch up events in one bunch without sending a sync event
in-between. However, the evdev_device struct only keeps track of one
pending touch up event so in this case the second touch up event would
override the first and it would be lost. This patch changes it to also
flush the events whenever the slot changes so that it will flush the
previous touch up event before trying to queue the next one.

https://bugs.freedesktop.org/show_bug.cgi?id=67563