fixes bug when setting env variable (#170)

samples/videoDecodePerf/videodecodeperf.cpp

@@ -163,7 +163,7 @@ int main(int argc, char **argv) {
         std::string gcn_arch_name_base = (pos != std::string::npos) ? gcn_arch_name.substr(0, pos) : gcn_arch_name;
 
         // gfx90a has two GCDs as two separate devices 
-        if (!gcn_arch_name_base.compare("gfx90a")) {
+        if (!gcn_arch_name_base.compare("gfx90a") && num_devices > 1) {
             sd = 1;
         }