D0i3 is bus power saving feature. It involves the
firmware - the driver needs to send a list of commands
to the firmware before entering this state. Wake up from
d0i3 also requires a few commands to the firmware.
The trigger to enter D0i3 is an idle timeout that will be
implemented later and will most probably rely on RUNTIME_PM
infrastructure.
In order to prevent entrance to D0i3 in critical flows, we
implement here a reference infrastructure. When a ref is
taken, we can't enter D0i3.