intel_extreme: Add RC6 downclocking support

* Generation 6 (SandyBridge) and later support
automatic downclocking of the GPU offering
substantial battery use reductions.
* As we're playing with fire here, only use on
mobile devices SandyBridge or later.
* This is testing stable on my SandyBridge laptop,
however I need further confirmation of the
functionality of this.
* Move clock gating into a function in the power.cpp
file