Cycles: require Experimental to be set to enable CUDA on cards with shader model

lower than 1.3, since we're not officially supporting these. We're already not
providing CUDA binaries for these, so better make it clear when compiling from
source too.

intern/cycles/device/device_multi.cpp

@@ -132,10 +132,10 @@ public:
 		return desc.str();
 	}
 
-	bool load_kernels()
+	bool load_kernels(bool experimental)
 	{
 		foreach(SubDevice& sub, devices)
-			if(!sub.device->load_kernels())
+			if(!sub.device->load_kernels(experimental))
 				return false;
 
 		return true;