HID: hid-multitouch: Remove the redundant touch state

With the input_mt_sync_frame() function in place, there is no longer
any need to keep the full touch state in the driver. This patch
removes the slot state and replaces the lookup code with the input-mt
equivalent.

Reviewed-and-tested-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>