GN build: replace magma.gni vars in Lavapipe

This was missed in the previous CL; Lavapipe wasn't built in CQ.

Change-Id: I4f1c016c0fa616cd527a97f7601031659729f1c8
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1029238
Reviewed-by: Craig Stout <cstout@google.com>
Fuchsia-Auto-Submit: Josh Gargus <jjosh@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>