[server] Generate correct touch ids

Summary:
testWaylandSeat fails because the seat interface generates incorrect id
for the second touch.

Test Plan: testWaylandSeat passes.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D18628