drm/atomic-helper: Add private_obj atomic_create_state helper

drm/atomic-helper: Add private_obj atomic_create_state helper

Now that we have an atomic_create_state callback for drm_private_objs,
we can provide a helper for it.

It's somewhat different from the other similar helpers though, because
we definitely expect drm_private_obj to be subclassed. It wouldn't make
sense for a driver to use it as-is.

So we can't provide a straight implementation of the atomic_create_state
callback, but rather we provide the parts that will deal with the
drm_private_obj initialization, and we will leave the allocation and
initialization of the subclass to drivers.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260128-drm-private-obj-reset-v4-3-90891fa3d3b0@redhat.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>