[platforms/drm] Explicitly request event context version 2

Summary:
Libdrm 2.4.78 introduces a version 2 and if KWin gets built against it
our code would break. Given that this change is for Plasma/5.8 branch.

Closes T5839

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Maniphest Tasks: T5839

Differential Revision: https://phabricator.kde.org/D5380