When there are 2 fingers on the pad we don't know which one is which, so
use input_mt_assign_slots to make sure the right set of coordinates ends
up in the right slot.

Besides ensuring things end up in the right slot, this also results in a
nice cleanup, since sync_frame also handles non mt position and btn_touch
reporting.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>