[libinput] Ensure Event::device returns a proper Device

Summary:
This fixes a problem when a Device added and another event on the Device
are queued together. In that case the second event would not get the
Device set as it's not yet created.

This change ensures that when accessing device the pointer will be
updated.

BUG: 389674
FIXED-IN: 5.12.0

Reviewers: #kwin, #plasma, fvogt

Subscribers: plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10236