meson: a define when exactly one gallium driver is enabled
Now that !14541 (merged) has landed, there's no way to load zink other than the env var. It should, however, be the case that if zink is the only gallium driver built, then zink is automatically loaded. To do this, I think we'd want meson to detect the scenario and set a define that could be used to trigger the zink init codepath.