touchpad: lock the touchpad rotation to the tablet rotation

Follow-up to 6229df18
We must not rely on the caller to toggle the left-handed bits correctly since
they may not know which devices belong together (despite device groups). Let's
do the right thing here, if the tablet is set to left-handed, rotate the
touchpad accordingly.

Note that the left-handed setting of the tablet is left as-is
(right-handed). Until we have notifications about configuration changes, this
is the best we can do.

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