gpu screen infrastructure patches

[Dave Airlie]

Hi,

This set of patches introduces infrastructure to support GPU
screens, hotplugging of platform devices and provision of the
randr provider objects. This code is mostly scaffolding, and
by itself doesn't provider any new feature, except exposing
providers to the clients. The providers so far won't support
any abilities or roles yet, just protocol to show the exist.

So you can hotplug and watch them appear/disappear in the
randr protocol.

This set requires the randrproto changes sent to the list
along with the platform bus patches sent to the list.

Dave.