[rfc] drm/ttm/memcg: simplest initial memcg/ttm integration

[Dave Airlie <airlied@gmail.com>]

Hey,

I've been tasked to look into this, and I'm going start from hopeless
naivety and see how far I can get. This is an initial attempt to hook
TTM system memory allocations into memcg and account for them.

It does:
1. Adds memcg GPU statistic,
2. Adds TTM memcg pointer for drivers to set on their user object
allocation paths
3. Adds a singular path where we account for memory in TTM on cached
non-pooled non-dma allocations. Cached memory allocations used to be
pooled but we dropped that a while back which makes them the best target
to start attacking this from.
4. It only accounts for memory that is allocated directly from a userspace
TTM operation (like page faults or validation). It *doesn't* account for
memory allocated in eviction paths due to device memory pressure.

This seems to work for me here on my hacked up tests systems at least, I
can see the GPU stats moving and they look sane.

Future work:
Account for pooled non-cached
Account for pooled dma allocations (no idea how that looks)
Figure out if accounting for eviction is possible, and what it might look
like.

Dave.