[Intel-gfx] Allocate objects from stolen memory in preparation for BIOS takeover

[Chris Wilson]

In comparison to last time, the use of generic objects backed by stolen
memory is fully supported. This has been used to test stolen objects by
allocating the ringbuffer, fbcon and other permanent objects out of
stolen memory and using them in anger.

The first several patches are in preparation for constructing new types
of objects by removing the complication of extraneous tracking lists and
disassociating the link between GTT space and pinned pages. We then
separate out the current stolen handling from dma, and begin the process
of making the core code stolen aware. Finally we start using stolen
memory to create objects, with the last patch suggesting how we might
use the stolen object to preserve the BIOS framebuffer (and mode
configuration) across GEM initialisation.
-Chris