pm: device: remove usage of local states

The device PM subsystem already holds the device state, so there is no
need to keep duplicates inside the device. The pm_device_state_get has
been refactored to just return the device state. Note that this is still
not safe, but the same applied to the previous implementation. This
problem will be addressed later.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>