Bus drivers are responsible for enumerating, configuring, and controlling devices.
The specific driver for the device is responsible for saving and restoring device context, and requesting power state changes as the policy owner for the device. In some cases, there may be nothing to save. Doing so would cause the system to not work reliably. The device driver's responsibility is to perform only proprietary actions. The PCI Power Management registers would control the internal state of a device, such as internal clocks and power planes.