HID: compute an ordered list of input fields to process

This will be used in a later commit:
we build a list of input fields (and usage_index) that is ordered based
on a usage priority.

Changing the usage priority allows to re-order the processed list, meaning
that we can enforce some usages to be process before others.

For instance, before processing InRange in the HID tablets, we need to
know if we are using the eraser (side or button). Enforcing a higher
(lower number) priority for Invert allows to force the input stack to
process that field before.

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