Picking a crankset for a 107mm BB on a downhill bike
According to the tech sheet at http://www.commencal-store.co.uk/PBSCProduct.asp?ItmID=12450127 you need a Press-fit BB.

PF107 should have 41mm diameter cups, have the same overall width as an External-bearing BB in an 83mm shell, and be compatible with the same cranks as "normal" external-83mm BBs.

SM-BB71 should work in your frame, and allow you to fit FC-M815 cranks. The cups will be the same as the "73mm" pressfit BBs, but the inner cover (aka spacer tube) is longer for "83mm".