drm/gem: Export implementation of shadow-plane helpers

Export the implementation of duplicate, destroy and reset helpers for
shadow-buffered plane state. Useful for drivers that subclass struct
drm_shadow_plane_state.

The exported functions are wrappers around plane-state implementation,
but using them is the correct thing to do for drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210705074633.9425-2-tzimmermann@suse.de