Support AMD Radeon PRO W6800 on Mac Metal (standalone, not duo)

Premiere Pro supports the AMD Radeon PRO W6800 for hardware acceleration on Windows OpenCL, but it does not support it on Mac Metal. It *does* support the W6800X Duo on Mac Metal, as that version ships with Mac Pros, but it does not support the standalone version, which you need to use if you are on a MacBook + eGPU setup.