drm/i915: Include bound and active pages in the count of shrinkable objects

When the machine is under a lot of memory pressure and being stressed by
multiple GPU threads, we quite often report fewer than shrinker->batch
(i.e. SHRINK_BATCH) pages to be freed. This causes the shrink_control to
skip calling into i915.ko to release pages, despite the GPU holding onto
most of the physical pages in its active lists.

References: https://bugs.freedesktop.org/show_bug.cgi?id=72742
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Robert Beckett <robert.beckett@intel.com>
Reviewed-by: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>