vulkan: only use M-sized matmul on Apple GPUs (#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 <slp@redhat.com>

* 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 <slp@redhat.com>

---------

Signed-off-by: Sergio Lopez <slp@redhat.com>