[AMDGPU] Common up default value of -amdgpu-nsa-threshold. NFC.

The default value of 3 was specified in two places. Use the actual value
of the cl::init to avoid repeating it.

llvm/lib/Target/AMDGPU/GCNSubtarget.cpp

@@ -708,7 +708,7 @@ unsigned GCNSubtarget::getNSAThreshold(const MachineFunction &MF) const {
   if (Value > 0)
     return std::max(Value, 2);
 
-  return 3;
+  return NSAThreshold;
 }
 
 GCNUserSGPRUsageInfo::GCNUserSGPRUsageInfo(const Function &F,