drm/i915: Free batch pool when idle

At runtime, this helps ensure that the batch pools are kept trim and
fast. Then at suspend, this releases memory that we do not need to
restore. It also ties into the oom-notifier to ensure that we recover as
much kernel memory as possible during OOM.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>