drm/ast: Move all of the cursor-update functionality to atomic_update

We used to update the cursor image in prepare_fb. Move all this code to
atomic_update (where it belongs). The generic helper for shadow-buffered
planes now implement the cursor plane's prepare_fb.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-11-tzimmermann@suse.de