CUDA: re-use MLA K data for V in MMA FA (#19057)

ggml/src/ggml-cuda/fattn.cu

@@ -247,6 +247,8 @@ static best_fattn_kernel ggml_cuda_get_best_fattn_kernel(const int device, const
         }
     }
 
+    const bool V_is_K_view = V->op == GGML_OP_VIEW && V->src[0] == K && V->data == K->data;
+
     const int cc = ggml_cuda_info().devices[device].cc;
 
     switch (K->ne[0]) {
@@ -269,6 +271,9 @@ static best_fattn_kernel ggml_cuda_get_best_fattn_kernel(const int device, const
             if (!gqa_opt_applies || gqa_ratio % 4 != 0) {
                 return BEST_FATTN_KERNEL_NONE;
             }
+            if (!V_is_K_view) {
+                return BEST_FATTN_KERNEL_NONE;
+            }
             break;
         default:
             return BEST_FATTN_KERNEL_NONE;