[platforms/drm] Move cursor dumb buffers to Output

Summary:
So far all outputs shared the same dumb buffer for the cursor image.
This doesn't work any more when screen rotation is enabled. For rotated
screens the cursor image per output is different. On some it might be
rotated, on some not.

To solve this problem the dumb buffers are moved from the DrmBackend to
the DrmOutput. The DrmOutput now creates the cursor images itself and
can rotate them if needed. Thus we get nicely transformed cursors.

Test Plan: Rotated screens, moved cursor around, image properly rotated

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D8664