flash: Fix loop iteration for gemmini

Kernel is software-pipelined around 2 GEMMs and softmax; it requires two
iterations to fully complete a tile.

tests/regression/flash_attention/kernel.gemmini.cpp

@@ -336,7 +336,11 @@ void kernel_body(int task_id, kernel_arg_t *__UNIFORM__ arg) {
   // "inner loop" along the columns of K^T
   const uint32_t k_tiles = (dim_seqlen / B_COL);
   for (uint32_t tile_k = 0;
-       tile_k < (4 /*for perf measurement*/ * k_tiles) + 2 /*pipeline latency*/;
+       tile_k < (4 /*for perf measurement*/ *
+                 // virgo kernel is fully pipelined around (2 GEMMs | softmax);
+                 // requires two loop iterations to finish one tile compute
+                 (2 * k_tiles)) +
+                    2 /*pipeline latency*/;
        tile_k++) {
     if constexpr (DEBUG || true) {
       threadblock_barrier(global_barrier_id, warps_per_threadblock_per_core);