libinput: note if calibrating without an output

Print a note that calibration got skipped if the input device supports a
calibration matrix but there is no associated output to compute it from.
Helps with debugging touchscreen calibration issues.

The code is reorganized and commented a bit, but this does not change
the behaviour.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>