CUDA / sm_20:
* Enable Lamp MIS again, it works on a GTX 570 (which is sm_20) with latest SVN. Tested by vitos1k in IRC:.
Feedback from GTX 580 users would be appreciated on this. 

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54362 954f8c5b-7b00-dc11-b283-0030488c597c