[platforms/drm] Fix cursor output rotation
Summary:
The calculation of cursor position and orientation on outputs rotations other
than 0° and 180° was faulty.
This patch fixes both.
Test Plan: In live session cursor looks and moves as expected.
Reviewers: KWin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D25909