graphics/mesa-devel: allow LLVM=off with AMD GPUs after 2ce6ce9f7525

llvmpipe/lavapipe are not included in this port to avoid downgrade bugs,
so no need to create a separate option.

https://gitlab.freedesktop.org/mesa/mesa/-/commit/2aaa6ebba186
(cherry picked from commit 58c2c17aaa7f5c1ce5a8b5987c0c1f9fc298ab77)