[mlir][nvgpu] Fix packing accumlator matrix (#69316)

The #68728 significantly simplified the accumulator matrix type, making
it easier to work with the nvgpu dialect without worrying about the
number of required structs, as this information is abstracted away in
the nvgpu-to-nvvm transformation.

However, we forgot packing the structs after initialization, causing the
accumulator matrix to hold undefined values, which is wrong. This PR
addresses that.