v3dv/pipeline: use a array instead of individual pointer to stages

Before this commit we were using individual pointers to each pipeline stage struct. We did that instead of an array because we needed to had a pointer for the binner stages too, and at that time we didn't have a enum to handle those stages.

Since then we introduced broadcom_shader_stage, and started to use in a lot of places (and per-stage arrays) so we can now use an array.

The main advantage is being able to handle several cases as loops. This also adds some consistency to the code (because as mentioned, in a lot of other places we use an array).