ACPI / PM: Drop acpi_bus_get_power()

There are no more users of acpi_bus_get_power(), so it can be
dropped.  Moreover, it should be dropped, because it modifies
the device->power.state field of an ACPI device without updating
the reference counters of the device's power resources, which is
wrong.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>