gallium,util: Pull u_indices and u_primconvert back into gallium

This was moved in !13741 (merged) but doing so created a link-time dependency between util and gallium which causes problems for Vulkan drivers. Meanwhile, having mesa/main depend on gallium is fine now that we don't have any classic drivers. It's a bit circular but should be harmless.

Fixes: 97ba2f2f ("move util/indices to core util") Closes: #8098 (closed)