drm/i915: Only attempt to scan the requested number of shrinker slabs

Since commit 4e773c3a ("drm/i915: Wire up shrinkctl->nr_scanned"),
we track the number of objects we scan and do not wish to exceed that as
it will overly penalise our own slabs under mempressure. Given that we
now know the target number of objects to scan, use that as our guide for
deciding to shrink as opposed to the number of objects we manage to
shrink (which doesn't correspond to the numbers we report to shrinkctl).

Fixes: 4e773c3a ("drm/i915: Wire up shrinkctl->nr_scanned")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115212455.24046-2-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
(cherry picked from commit 29d384e3)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>