drm/dp_mst: Switch private_obj initialization to atomic_create_state

drm/dp_mst: Switch private_obj initialization to atomic_create_state

The DP MST implementation relies on a drm_private_obj, that is
initialized by allocating and initializing a state, and then passing it
to drm_private_obj_init.

Since we're gradually moving away from that pattern to the more
established one relying on a atomic_create_state implementation, let's
migrate this instance to the new pattern.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://patch.msgid.link/20260128-drm-private-obj-reset-v4-5-90891fa3d3b0@redhat.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>