graph : create V as a view of K in the k_iswa build_attn (#27392)

build_attn with the llm_graph_input_attn_k_iswa input was using the cached K
tensor itself as V. Create V as a view of K (the first v_cur->ne[0] elements
of each row), like the other K-only build_attn overloads.

The deepseek4 MTP call site now passes the kv tensor as v_cur.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

src/models/deepseek4.cpp

@@ -1225,7 +1225,7 @@ ggml_tensor * llama_model_deepseek4::graph::build_attention_impl(
     if (inp_mtp) {
         out = build_attn(inp_mtp,
                 nullptr, nullptr, nullptr,
-                q, kv, nullptr,
+                q, kv, kv,
                 nullptr, layer.attn_sinks, nullptr,
                 1.0f/sqrtf(float(n_embd_head)), il);
         cb(out, "attn_raw", il);