ac/llvm: remove the num_channels parameter from ac_build_buffer_store_dword

ac/llvm: remove the num_channels parameter from ac_build_buffer_store_dword

It was used when LLVM didn't support vec3 and we had to pass vec4
with num_channels=3. We no longer need to do that.

This also removes the vec3 splitting or conversion to vec4 in callers.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14266>