Input: alps - improve 2-finger reporting on v3 models

V3 models only report mt bitmap data when there are 2 or more fingers on
the touchpad. So always generate 2 positions in alps_process_bitmap, and
for v3 models only fall back to st data when there was no mt data in a
mt packet (which should never happen).

This fixes 2 finger scrolling not working when using 2 fingers close to
each other.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>