XTest devices are the first ones in the list, being initialised together
with the master devices. If we disable the devices in-order and a device has
a button down when being disabled, the XTest device is checked for a
required button release (xkbAccessX.c's ProcessPointerEvent). This fails if
the device is already NULL.

Instead of putting the check there, disable the devices in the reverse order
they are initialised. Disable physical slaves first, then xtest devices,
then the master devices.

Testcase: shut down server with a button still held down on a physical
device

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>