drm/ast: Store cursor BOs in cursor plane

The cursor uses two BOs in video RAM to implement double buffering. Store
both in struct ast_cursor_plane.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-8-tzimmermann@suse.de