[lavapipe] Separate Magma and Fuchsia build deps in Mesa.

- Start of decoupling that will need to happen in multiple places for
  Lavapipe to work.

- Fixed build error.

Change-Id: I15124e3d047fe6b953e56d800fd086809e2fa8d6
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/954472
Commit-Queue: John Rosasco <rosasco@google.com>
Reviewed-by: John Bauman <jbauman@google.com>
7 files changed