vulkan: only use M-sized matmul on Apple GPUs (llama/5412)

vulkan: only use M-sized matmul on Apple GPUs (llama/5412)

* vulkan: refactor guess_matmul_pipeline for vendor

Refactor ggml_vk_guess_matmul_pipeline to simplify adding per-vendor
conditionals.

Signed-off-by: Sergio Lopez <redacted>
* vulkan: only use M-sized matmul on Apple GPUs

L-sized and S-sized matmuls are broken on Apple GPUs, force using
M-size with this vendor.

Signed-off-by: Sergio Lopez <redacted>
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Signed-off-by: Sergio Lopez <redacted>