[Intel-gfx] ppGTT the recursive wars

[Chris Wilson]

Doing more with less!

We restore and succesfully generalise a recently lost optimisation to
avoid updating page directories to be removed, all while using less i$
and hopefully more predictable branches. Mircoptimisations, but less
code for the win.

We even add some debug traces for the next poor soul to be trapped here.

drivers/gpu/drm/i915/Kconfig.debug  |  15 +
drivers/gpu/drm/i915/i915_gem_gtt.c | 586 +++++++++++++++---------------------
drivers/gpu/drm/i915/i915_gem_gtt.h |  92 +-----
3 files changed, 259 insertions(+), 434 deletions(-)