drm/ast: Unpin cursor BO during cleanup

The unpin operation was missing from ast_cursor_fini(). Fixed now.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613073041.29350-3-tzimmermann@suse.de