touchpad: increase top software button area to 15%

touchpad: increase top software button area to 15%

We had reports that the top software button area is hard to hit for those
using the trackpoint and clicking the buttons with their thumb.

Analysis of event recordings (3 different people) for left, right and middle
clicks shows that there is a significant amount of events up to about 10mm
(with outliers up to 12mm) from the top of the touchpad. That maps to 15%.

Interestingly, the middle button is not affected by this, presumably the
haptic feedback of the little dots sticking out from the surface make hitting
the button easier.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Hans de Goede <hdegoede@redhat.com>