drm/atomic-helper: Add format-conversion state to shadow-plane state

Store an instance of struct drm_format_conv_state in the shadow-plane
state struct drm_shadow_plane_state. Many drivers with shadow planes
use DRM's format helpers to copy or convert the framebuffer data to
backing storage in the scanout buffer. The shadow plane provides the
necessary state and manages the conversion's intermediate buffer memory.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231009141018.11291-3-tzimmermann@suse.de