ggml-vulkan: enable flash-attn mask_opt for GCN large head sizes

mask_opt was disabled on AMD GCN for head sizes <= 256, but it is
beneficial there in high-context prefill: on fully-visible causal
blocks it skips the per-block mask load+add, and it skips fully-masked
blocks entirely. The attention op on GCN is compute-bound on the
softmax path (no matrix cores), so this cuts real work.

Verified lossless (perplexity bit-identical with it on vs off) and a
measured prefill win on Qwen3.5-35B-A3B (head_dim 256) on an RX 580:
pp2048 unchanged at short context, +8% @ 16k, +12% @ 32k, growing with
depth. Enable for GCN when HSK/HSV >= 256; the existing large-mask
conditions keep it off for decode.

Assisted-by: Claude

ggml/src/ggml-vulkan/ggml-vulkan.cpp

@@ -10592,8 +10592,10 @@ static void ggml_vk_flash_attn(ggml_backend_vk_context * ctx, vk_context& subctx
     }
 
     // Only use mask opt when the mask is fairly large. This hasn't been tuned extensively.
+    // GCN with large head size (>= 256) benefits in high-context prefill (skipping the
+    // per-block mask add on fully-visible blocks dominates), so enable it there too.
     bool use_mask_opt = mask && nem1 >= 32 && nem0 * nem1 > 32768 && nem0 >= tuning_params.block_cols * 16
-                        && (ctx->device->architecture != vk_device_architecture::AMD_GCN || HSK > 256 || HSV > 256);
+                        && (ctx->device->architecture != vk_device_architecture::AMD_GCN || HSK >= 256 || HSV >= 256);
     vk_fa_pipeline_state fa_pipeline_state = get_fa_pipeline_state(ctx->device, tuning_params, HSK, HSV, aligned, f32acc,
                                                                    mask != nullptr, use_mask_opt, logit_softcap != 0, k->type, v->type);