power: device_pm: Remove device idle pm states

Avoid confusion with device runtime idle pm states and just use device
pm states.

This simplify the code a little bit and prepare the ground for having
a better definition of device pm states. Right now this code needed to
hijack two transitional states to not break the current code logic but
the goal is avoid it and have everything in one single place.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>