drm/i915: Use topdown allocation for PPGTT PDEs on gen6/7

It was always the intention to do the topdown allocation for context
objects (Chris' idea originally). Unfortunately, I never managed to land
the patch, but someone else did, so now we can use it.

As a reminder, hardware contexts never need to be in the precious GTT
aperture space - which is what is what happens with the normal bottom up
allocation we do today. Doing a top down allocation increases the odds
that the HW contexts can get out of the way, especially with per FD
contexts as is done in full PPGTT

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>