ACPI: Add a convenience function to tell a device is in D0 state

Add a convenience function to tell whether a device is in D0 state,
primarily for use in drivers' probe or remove functions on busses where
the custom is to power on the device for the duration of both.

Returns false on non-ACPI systems.

Suggested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>