[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