wacom: Fix small memleak
wacom: Add method to get the default calibration values

From the device's valuators.
wacom: Implement resetting the tablet area to default

The Wacom driver doesn't actually understand negative values as
meaning "the default values" so we were trashing the defaults
as soon as we modified the tablet area.

https://bugzilla.gnome.org/show_bug.cgi?id=707784