Fix #34921: cycles rendered a specific scene with a small high poly object
contained in a large low poly object very slow, due to failure case of fast
multithread BVH binning. Tweaked parameter now to avoid this.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55912 954f8c5b-7b00-dc11-b283-0030488c597c