drm/atomic: Add new atomic_create_state callback to drm_private_obj

drm/atomic: Add new atomic_create_state callback to drm_private_obj

The drm_private_obj initialization was inconsistent with the rest of the
KMS objects. Indeed, it required to pass a preallocated state in
drm_private_obj_init(), while all the others objects would have a reset
callback that would be called later on to create the state.

However, reset really is meant to reset the hardware and software state.
That it creates an initial state is a side-effect that has been used in
all objects but drm_private_obj. This is made more complex since some
drm_private_obj, the DisplayPort ones in particular, need to be
persistent across and suspend/resume cycle, and such a cycle would call
drm_mode_config_reset().

Thus, we need to add a new callback to allocate a pristine state for a
given private object.

This discussion has also came up during the atomic state readout
discussion, so it might be introduced into the other objects later on.

Until all drivers are converted to that new allocation pattern, we will
only call it if the passed state is NULL. This will be removed
eventually.

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-v4-2-90891fa3d3b0@redhat.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>