drm/i915: Mark up vma->active as safe for use inside shrinkers

Since a shrinker may be forced to wait on GPU activity,
i915_active_wait(&vma->active) must be safe for use inside a shrinker,
and so let's mark up the lock as being acquired by the shrinker to avoid
any nasty surprises creeping in.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703091726.11690-8-chris@chris-wilson.co.uk