Use the buffer mappings provided by shadow-plane helpers. As the
mappings are established while the commit can still fail, errors
are now reported correctly to callers.

v2:
	* use shadow-plane state directly (Noralf)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org> # drm/tiny/mi0283qt
Link: https://patchwork.freedesktop.org/patch/msgid/20221202125644.7917-8-tzimmermann@suse.de