drm/ast: Add cursor-plane data structure

Cursor state is currently located throughout struct ast_private. Having
struct ast_cursor_plane as dedicated data structure for cursors helps to
organize the modesetting code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-7-tzimmermann@suse.de