tu: Fix border color with compute shaders

tu: Fix border color with compute shaders

I wasn't able to find any CTS tests that used compute shaders with
samplers and set a border color, so I hacked one of the tests included
with amber:

https://gist.github.com/cwabbott0/e72f0ed8259b84ed6bf3920c68fefee6

The register was found via looking at dumps of the Vulkan blob, and
setting it fixes this test.

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4204>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4204>