[mesa] Add comment for overloaded parameter.

Change-Id: Ia131cc7e3f9509177ede8ed33c7f78f6b27f5929
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/991872
Reviewed-by: Craig Stout <cstout@google.com>
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1005915
Commit-Queue: Josh Gargus <jjosh@google.com>
Reviewed-by: Alice Neels <neelsa@google.com>
Reviewed-by: Josh Gargus <jjosh@google.com>