vulkan: fix min subgroup 16 condition for mmid subgroup optimization

[0cc4m]

This fixes a bug in the selection of the subgroup mmid optimization introduced in #15524.

@MrLavender please give it a try, it should be working for you with this fix.

[MrLavender]

Yes that works, thank you! :)

This is a fantastic win because with flash attention the ROCm backend is now only better than Vulkan at small pp sizes where the difference doesn't really matter, and ROCm performance degrades very quickly as pp size increases.

llama-bench -fa 0,1 -p 512,1024,2048,4096 -m gpt-oss-20b-mxfp4.gguf 

Vulkan

model	size	params	backend	ngl	fa	test	t/s
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	0	pp512	976.97 ± 8.17
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	0	pp1024	969.50 ± 1.57
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	0	pp2048	910.61 ± 1.93
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	0	pp4096	849.46 ± 2.31
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	0	tg128	125.82 ± 0.05
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	1	pp512	980.48 ± 4.63
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	1	pp1024	975.67 ± 7.09
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	1	pp2048	961.13 ± 3.07
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	1	pp4096	929.77 ± 3.94
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	Vulkan	99	1	tg128	123.70 ± 0.10

ROCm

model	size	params	backend	ngl	fa	test	t/s
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	0	pp512	1712.99 ± 14.24
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	0	pp1024	1652.80 ± 4.07
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	0	pp2048	1537.22 ± 4.35
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	0	pp4096	1376.64 ± 2.84
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	0	tg128	95.96 ± 0.19
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	1	pp512	1244.60 ± 3.71
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	1	pp1024	1078.54 ± 3.17
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	1	pp2048	884.51 ± 2.27
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	1	pp4096	640.66 ± 0.59
gpt-oss 20B MXFP4 MoE	11.27 GiB	20.91 B	ROCm	99	1	tg128	93.37 ± 0.25