Changes required to avoid meson.build errors after commit b8b38d38
Fixes: b8b38d38 ("meson: reinstate LLVM requirement for r300 and enforce it for i915 too")
Reviewed-by: Valentine Burley <valentine.burley@collabora.com>
Reviewed-by: Antonio Ospite <antonio.ospite@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37694>