Input: Revert "Revert "synaptics - use dmax in input_mt_assign_slots""

This reverts commit 09d042a2

 ("Revert "Input: synaptics - use dmax in
input_mt_assign_slots"")

Now that balanced slots assignments seem to be fixed, let's re-enable the
use in synaptics.c and wait for users to complain if there are still
problems.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Henrik Rydberg <rydberg@bitmath.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>