[fuchsia] Fix TODO formatting.

Also, improve consistency of comment style (/* vs //).

Change-Id: I872f1d9ecf729b591b36bb42a4173ded5e3affbb
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1016100
Reviewed-by: John Rosasco <rosasco@google.com>
Commit-Queue: Josh Gargus <jjosh@google.com>