tu: Rewrite border color handling

Emit a single table of all possible Vulkan border colors up front, and then index into it using the Vulkan enum directly. In fact this seems to be the entire point of separating out border colors in the first place.

In addition to being simpler and having less CPU overhead, and fixing cases where more than one sampler uses border color, this paves the way for bindless samplers because the existing approach isn't great for bindless.