v3dv: meta operations can happen outside a render pass

We were asserting that we had a valid subpass index, but we can have
meta operations that run outside a render pass, such as for blitting.

If we allow this, then we also need to account for the fact that
pipelines can be bound outside a render pass too.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>
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