Clean up SDMA ganging

We don't need to keep track of specific blit engines in gang for
submission anymore as ganging early exits on pending bytes.
So tidy up the fluff.

Change-Id: I77e80bf1ad8f561a03fff77bce33aa09d02760c6

runtime/hsa-runtime/core/runtime/amd_topology.cpp

@@ -376,7 +376,6 @@ void BuildTopology() {
         continue;
 
       auto linfo = core::Runtime::runtime_singleton_->GetLinkInfo(src_id, dst_id);
-      GpuAgent *gpu = (AMD::GpuAgent*)src_gpu;
       // xGMI link type cannot determine bandwidth keep it fixed for ganging
       bool has_fixed_gang = linfo.info.link_type == HSA_AMD_LINK_INFO_TYPE_XGMI &&
                             linfo.info.numa_distance != 15;
@@ -392,7 +391,7 @@ void BuildTopology() {
       uint32_t gang_factor = has_fixed_gang ? 2 : (linfo.info.min_bandwidth ?
                              linfo.info.max_bandwidth/linfo.info.min_bandwidth : 0);
 
-      gpu->RegisterGangPeer(*dst_gpu, gang_factor);
+      ((AMD::GpuAgent*)src_gpu)->RegisterGangPeer(*dst_gpu, gang_factor);
     }
   }
 }