Associating input devices with weston_outputs by the output name fails
when one output has several heads. We need to match against head names
instead of output names to be able to find all names.

This fixes touchscreen output association in shared-CRTC clone mode when
outputs or input devices appear or disappear.

Even though notify_output_create() is called only when new outputs
appear, the implementation is prepared to also remove output
associations. This will be handy in the future when this function will
handle also head detaching from an output that remains enabled.

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