[platforms/drm] Fix software cursors with drm backend

Summary:
If hardware cursor support is not available when using the drm backend for
Wayland compositing, the software cursor texture will not be updated when the
cursor image changes, and it will still be drawn when no cursor image is set
(such as when running a full-screen game). Furthermore, the drmModeSetCursor
and drmModeMoveCursor functions will still be unnecessarily called when the
cursor is moved or hidden.

To correct this, SceneOpenGL should connect Platform::cursorChanged as opposed
to Cursor::cursorChanged to its texture update function, as only the former
will be emitted when the cursor is updated and the compositor should check if
the cursor is hidden and the software cursor image is not null before rendering
it. DrmBackend::moveCursor and DrmBackend::hideCursor should also return
immediately if using a software cursor.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18376