[mlir][nvgpu] NFC - move NVGPU conversion helpers to NvGpu utils library

The ConvertVectorToGpu pass implementation contained a small private
support library for performing various calculations during conversion
between `vector` and `nvgpu.mma.sync` and `nvgpu.ldmatrix` operations.
The support library is moved under `Dialect/NVGPU/Utils` because the
functions have wider utility. Some documentation comments are added or
improved.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D135303