Input: synaptics - initialize pointer emulation usage

To properly setup event parameters for emulated events, pass
the appropriate flag to the slot initialization function. Also,
all MT-related events should be setup before initialization.

Incidentally, this solves the issue of doubly filtered pointer
events.

Reported-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>