The previous approach only had the slot state for the current slot. If we
changed slots, that means we lost the information if the slot was ever
initialized. If the ABS_MT_TRACKING_ID was never received, the slot would
still update and try to send events (which the server refused with a warning).

Avoid this by having a per-slot state and a dirty bit that tells us if the
current slot updated at all. If we don't get the tracking ID, leave the slot
empty and refuse any further events from that touch.

This quashes the various "unable to find touch point 0" warnings caused if a
touchpoint starts before the device is enabled.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Walter Harms <wharms@bfs.de>