[Intel-gfx] Patch set to enable cache shrinking

[Chris Wilson]

Hi guys, this is just a dump of my outstanding patches, plus an initial
attempt at implementing purgeable objects. The interface is quite simple
and borrows heavily from madvise DONTNEED/WILLNEED, will a single return
value indicating whether the backing pages still exist. This can then be
used by user-space to mark caches as purgeable and lazily update them
under memory pressure.