storeLastValuators() takes the index in the mask for the x and y axis.
Completely pointless because any device that doesn't have x/y on 0 and
1, respectively, is going to break in fun ways anyway. And we only have
two callers two this function, both of which hardcode 0 and 1.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>