fallback: force the palm state to PALM_NONE on touch begin

If we don't have tool-based palm detection, make sure our touch is labelled as
"not palm" during touch down. Otherwise that slot remains on palm forever if
it gets tagged as palm through some other means.

This currently has no effect, nothing in the code would label the touch as
palm. This is prep work for better touch arbitration.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>