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.

Tested-by: Marge Bot <mesa/mesa!4200>
Part-of: <mesa/mesa!4200>