Input: uinput - setup MT usage during device creation

The input devices created by uinput do not currently handle multitouch
properly. All events will appear as if they came from slot zero, and
the input event buffers are not adjusted. This patch creates the MT
slots during setup, and sets the number of events per packet based
on the MT usage.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>