[Intel-gfx] Hopefully the last round of stolen work

[Chris Wilson]

So nearly everything has been reviewed, thanks mostly to Ben and Jesse.
Both Ben and Paulo have tested the patches, and a smattering of bug
reporters have also unbeknownst to them also provided testing feedback.
It has been running on all of my machines for several months. The
stolen handling is a prerequisite for fastboot.

The big changes since last time are that a few patches have been
reworked to hopefully make Ben happier, notably
  03/16: Fix detection of base of stolen memory
  05/16: Delay allocation of stolen space for FBC
and there was the realisation that there was one mmu-notifier that was
called outside of the mm locks which we could use to sleep on GPU
activity before page tables were modified. Hence, provided the
mmu-notifier is present, we can do userptr safely for normal users.
-Chris