drm/ast: Store each HW cursor offset after pinning the rsp BO

As HW cursor BOs never move, we can store the offset in VRAM in the
cursor-plane's HWC state. This removes the last possible source of
runtime errors from atomic_update.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-10-tzimmermann@suse.de