[MLIR] change NVVM.mma.sync to the most useful variant.

Summary:
the .row.col variant turns out to be the popular one, contrary to what I
thought as .row.row. Since .row.col is so prevailing (as I inspect
cuDNN's behavior), I'm going to remove the .row.row support here, which
makes the patch a little bit easier.

Reviewers: ftynse

Subscribers: jholewinski, bixia, sanjoy.google, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74655