Because of the way in which we're allocating the pages for the Aliasing
PPGTT, we cannot actually successfully alloc enough space for anything
greater than 2GB.

Instead of a quick hack to fix this, we should defer until we have the
real solution in place (allocating much less contiguous space).

This wasn't found sooner because we didn't not have any systems
supporting more than a 2GB GTT.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>