drm/ast: Inline ast cursor-update functions into modesetting code

The logic for cursor updates is now located in the cursor plane's
modesetting code. A number of helper functions remain to modify the
rsp registers and image.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-6-tzimmermann@suse.de