[mlir] Fix -Wsign-compare in NVGPUDialect.cpp (NFC)

/llvm-project/mlir/lib/Dialect/NVGPU/IR/NVGPUDialect.cpp:396:31: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int64_t' (aka 'long long') [-Werror,-Wsign-compare]
  if (getCoordinates().size() !=
      ~~~~~~~~~~~~~~~~~~~~~~~ ^
1 error generated.