HID: multitouch: remove one copy of values

The current way of handling multitouch data is not very straightforward:
- in mt_event() we do nothing
- in mt_report() we:
  - do some gym to fetch the scantime and the contact count
  - then iterate over the input fields where we copy the data to a
    temporary place
  - when we see the last field in a slot, we then use this data to emit
    the input data

A more streamlined way is to first get all of the address in the report
of all fields, and then just pick the fields we are interested in in
mt_report()

Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>