drm/ast: Pass struct ast_private instance to cursor init/fini functions

Removes some typecasting.

v2:
	* use to_ast_private() instead of struct drm_device.dev_private

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-3-tzimmermann@suse.de