If INPUT_PROP_DIRECT is set, userspace doesn't have to fall back to old
ways of identifying touchscreen devices.

In order to identify a touchscreen device, Android for example, seems to
already depend on INPUT_PROP_DIRECT to be present in drivers. udev still
checks for either BTN_TOUCH or INPUT_PROP_DIRECT. Checking for BTN_TOUCH
however can quite easily lead to false positives; it's a code that not only
touchscreen device drivers use.

According to the documentation, touchscreen drivers should have this
property set and in order to make life easy for userspace, let's set it.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>