zink: only enable KHR_external_memory_fd if supported

Erik Faye-Lund requested to merge kusma/mesa:zink-swrast-no-dmabuf into master

While we're at it, make sure we error out if it's not supported when required.

This brings us a bit closer to being able to test on SwiftShader, which doesn't currently support KHR_external_memory_fd.

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