evdev: check for ABS_MT_POSITION_X/Y to determine mt devices

evdev: check for ABS_MT_POSITION_X/Y to determine mt devices

mtdev as currently used in weston is a noop. mtdev's purpose is to convert
Protocol A devices (without ABS_MT_SLOT) to Protocol B devices (slots).
For Protocol B devices mtdev merely routes the events, so checking for
slots and then using mtdev based on that adds no functionality.

Check for ABS_MT_POSITION_X/Y instead and use that to categorise a device
as MT device. mtdev will provide us with a slotted protocol for all devices.

https://bugs.freedesktop.org/show_bug.cgi?id=54428