drm/gem: Export helpers for shadow-buffered planes

Export the helpers for shadow-buffered planes. These will be used by
several drivers.

v3:
	* fix documentation typos and formatting (Daniel)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209121042.24098-2-tzimmermann@suse.de