[Intel-gfx] Shrinker improvements

[Chris Wilson]

Recent-ish changes to the core shrinker have made the code more prone to
hit OOM, and worse do so without even touching swap. My hypothesis is
that this is a balancing issue between shrinking the VM lists and the
slabs, such that writeback of our objects is not performed starving the
system of pages. Patch 5/5 introduces manual writeback handling for our
shrinker, but afaict it should be unnecessary. Nevertheless, I think it
will come in useful.

I am having difficulty in getting results from QA, but so far these have
improved mempressure handling on my machines.
-Chris