drm/i810: cleanup reclaim_buffers

My dear old i815 always hits the deadlocked on reclaim_buffers
warning. Switch over to the idlelock duct-tape on hope that
works better. I've fired up my i815 and now closing glxgears doesn't
take 5 seconds anymore. \o/

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>