meson: Refuse to build lavapipe without llvmpipe
This config doesn't work, and as of recently it no longer links. Let's fail it early.
Fixes: 7b79db11 ("lavapipe: enable correct workgroup sizing")
This config doesn't work, and as of recently it no longer links. Let's fail it early.
Fixes: 7b79db11 ("lavapipe: enable correct workgroup sizing")