drm/omap: omap_plane: subclass drm_plane_state

In preparation to add omap plane state specific extensions we need to
subclass drm_plane_state and add the relevant helpers.

The addition of specific extension will be done separately.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211117141928.771082-6-narmstrong@baylibre.com