[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