[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
Subscribers: plasma-devel, kwin
Tags: KWin
Maniphest Tasks: T5839
Differential Revision: https://phabricator.kde.org/D5380