the new LRU scan,
perhaps a small improvement... Except oddly for the poppler trace.

Reference:

  Bug 15911 - Intermittent X crash (freeze)
  https://bugzilla.kernel.org/show_bug.cgi?id=15911

  Bug 20152 - cannot view JPG in firefox when running UXA
  https://bugs.freedesktop.org/show_bug.cgi?id=20152

  Bug 24369 - Hang when scrolling firefox page with window in front
  https://bugs.freedesktop.org/show_bug.cgi?id=24369

  Bug 28478 - Intermittent graphics lockups due to overflow/loop
  https://bugs.freedesktop.org/show_bug.cgi?id=28478

v2: Attempt to clarify the logic and order of eviction through the use
of comments and macros.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>
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