drm/i915: Fix up usage of SHRINK_STOP

drm/i915: Fix up usage of SHRINK_STOP

In

commit 81e49f811404f428a9d9a63295a0c267e802fa12
Author: Glauber Costa <glommer@openvz.org>
Date:   Wed Aug 28 10:18:13 2013 +1000

    i915: bail out earlier when shrinker cannot acquire mutex

SHRINK_STOP was added to tell the core shrinker code to bail out and
go to the next shrinker since the i915 shrinker couldn't acquire
required locks. But the SHRINK_STOP return code was added to the
->count_objects callback and not the ->scan_objects callback as it
should have been, resulting in tons of dmesg noise like

shrink_slab: i915_gem_inactive_scan+0x0/0x9c negative objects to delete nr=-xxxxxxxxx

Fix discusssed with Dave Chinner.

References: http://www.spinics.net/lists/intel-gfx/msg33597.html
Reported-by: Knut Petersen <Knut_Petersen@t-online.de>
Cc: Knut Petersen <Knut_Petersen@t-online.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Glauber Costa <glommer@openvz.org>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>