HID: multitouch: Store per collection multitouch data

Currently, hid-multitouch can only handle one multitouch collection at
a time. This is an issue for the Dell Canvas, as the Totem (a dial tool)
is also using a multitouch-like collection.

Factor out the multitouch collection data in their own struct.

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>