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>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1035,8 +1035,7 @@ alloc:
 						  &ppgtt->node, GEN6_PD_SIZE,
 						  GEN6_PD_ALIGN, 0,
 						  0, dev_priv->gtt.base.total,
-						  DRM_MM_SEARCH_DEFAULT,
-						  DRM_MM_CREATE_DEFAULT);
+						  DRM_MM_TOPDOWN);
 	if (ret == -ENOSPC && !retried) {
 		ret = i915_gem_evict_something(dev, &dev_priv->gtt.base,
 					       GEN6_PD_SIZE, GEN6_PD_ALIGN,