PM: ACPI: Output correct message on target power state

Transitioned power state logged at the end of setting ACPI power.

However, D3cold won't be in the message because state can only be
D3hot at most.

Use target_state to corretly report when power state is D3cold.

Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>