ACPI / PM: Change the way power transitions to D3cold are carried out

During power transitions into D3cold from any shallower power states
we are supposed to transition the device into D3hot and remove power
from it afterward, but the current code in acpi_device_set_power()
doesn't work this way.

At the same time, though, we need to be careful enough to preserve
backwards compatibility for systems that don't distinguish between
D3hot and D3cold (e.g. designed before ACPI 4).

Modify acpi_device_set_power() so that it works in accordance with
the expectations in both cases.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>