[Intel-gfx] [RFC] Preserve stolen objects across initialisation

[Chris Wilson]

The goal of this series is to introduce a GEM object that is backend by
stolen memory and able to wrap preallocated objects before we have fully
initialised GEM. And then to keep those objects around as we do
intialise GEM for continued use afterwards.

This patchset is slightly larger than strictly required, as I thinking
of how to introduce stolen objects, I realised that it would be much
easier if I could base it on the list removal and disassociation between
obj->pages and obj->gtt_space.
-Chris