drm/i915/selftests: mark huge_gem_object as not shrinkable

It looks like some callers expect a non-volatile object, that they do not
want the contents of the pages lost if they happen to not be looking at it.
The shrinker however sees that we mark the pages as DONTNEED and
believes that it can freely reap them. However, since the huge object
use plain pages, they cannot be swapped out as they have no backing
storge, and the only way we can shrink them is by discarding the
contents. In light of the callers wanting to keep the contents around,
both IS_SHRINKABLE and marking the pages as volatile are incorrect.

If we drop the IS_SHRINKABLE flag we avoid the immediate issue of the
shrinker accidentally removing valuable content. We will have to
remember that a huge object is not suitable for exercising the shrinker
interaction -- although we can introduce a shrinkable one if we require.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323130821.47914-1-matthew.auld@intel.com