vulkan: optimize Q2_K and Q3_K mul_mat_vec

[jeffbolznv]

Similar to optimizations I did recently for Q{4-6}_K.

Perf results on RTX 4070:

before
| llama 8B Q2_K - Medium         |   2.95 GiB |     8.03 B | Vulkan     | 1000 |         tg128 |         59.12 ± 0.66 |
| deepseek2 16B Q2_K - Medium    |   5.99 GiB |    15.71 B | Vulkan     | 1000 |         tg128 |        102.23 ± 0.43 |

after
| llama 8B Q2_K - Medium         |   2.95 GiB |     8.03 B | Vulkan     | 1000 |         tg128 |         72.17 ± 0.51 |
| deepseek2 16B Q2_K - Medium    |   5.99 GiB |    15.71 B | Vulkan     | 1000 |         tg128 |        109.92 ± 0.62 |

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[0cc4m]

Looks good on Nvidia, AMD and Intel. Thank you.