drm/ast: Rename to_ast_private() to to_ast_device()

The helper to_ast_private() now upcasts to struct ast_device. Rename
it accordingly. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230221155745.27484-5-tzimmermann@suse.de