evdev: use filter_dispatch_constant() for button scrolling

evdev: use filter_dispatch_constant() for button scrolling

Our pointer filter code has two functions - one for accelerated movement
and one for "constant" movement (i.e. no accel factor provided but same
conversions). Let's use that instead of a manual normalization.

This fixes an issue with button scrolling on high-dpi mice in the flat
pointer acceleration: normal pointer motion in the flat profile isn't
normalized but the button scrolling was - resulting in e.g. 5 times
slower motion for button scrolling on a 5000dpi mouse.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>