Replace the confusing logic that had slightly different use of BM/BN/BK
for A and B, into one logic that accepts matrix memory layout as a
proper argument & does compile-time logic to determine the right
dimensions.
TODO: !GMEM_COALESCED_A is not updated yet
thread_block_gemm is meant to be reusable, so it shouldn't assume what
the kernel arg struct looks like.
threadblock_dim_y was ambiguous and didn't match the literal name either
(it was used as # of warps that participate in a barrier).
