For v3 protocol devices, use the more accurate single touch data when the
mt data contains only one finger. Note the mt data reporting a finger count
of 1 should never happen, but better safe then sorry.

This brings the v3 bitmap handling in line with what the v4 code does,
allowing to factor out the common bits into a helper function.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>