dix: scale y back instead of x up when pre-scaling coordinates

The peculiar way we handle coordinates results in relative coordinates on
absolute devices being added to the last value, then that value is mapped to
the screen (taking the device dimensions into account). From that mapped
value we get the final coordinates, both screen and device coordinates.

To avoid uneven scaling on relative coordinates, they are pre-scaled by
screen ratio:resolution:device ratio factor before being mapped. This
ensures that a circle drawn on the device is a circle on the screen.

Previously, we used the ratio to scale x up. Synaptics already does its own
scaling based on the resolution and that is done by scaling y down by the
ratio. So we can remove the code from the driver and get approximately the
same behaviour here.

Minor ABI bump, so we can remove this from synaptics.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Tested-by: Emmanuel Benisty <benisty.e@gmail.com>