drm/qxl: rework cursor plane

Add helper functions to create and move the cursor.
Create the cursor_bo in prepare_fb callback, in the
atomic_commit callback we only send the update command
to the host.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-11-kraxel@redhat.com