drm/ingenic: Add support for private objects

Until now, the ingenic-drm as well as the ingenic-ipu drivers used to
put state-specific information in their respective private structure.

Add boilerplate code to support private objects in the two drivers, so
that state-specific information can be put in the state-specific private
structure.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20211026181240.213806-3-paul@crapouillou.net


Tested-by: Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Christophe Branchereau <cbranchereau@gmail.com>