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.