dix: fix wheel emulation lockup when a negative increment is set

The increment sign wasn't taking into account when checking if the next value is past our current value. The result was that for negative increments, we kept looping indefinitely, locking up the server.

Easiest to reproduce with the evdev driver which has a negative increment on the y axis.

Fixes 0a22502c dix: switch scroll button emulation to multiples of increment

cc @ofourdan

Follow-up to !1059 (merged)