[AMDGPU] Fix -Wunused-variable in AMDGPUAsmPrinter.cpp (NFC)

/llvm-project/llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp:653:13:
error: unused variable 'PGMRSrc3' [-Werror,-Wunused-variable]
    int64_t PGMRSrc3;
            ^
1 error generated.