drm/ast: Managed cursor release

Register a release function to finalize cursors. The _fini() function
gets un-exported from the source file.

The function ast_mode_fini() is now empty and will be removed by a
later patch.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-11-tzimmermann@suse.de