sfxge(4): fix warnings from VS2015 C compiler (C4100)

Fix multiple level 4 warnings
"C4100: 'xxx': unreferenced formal parameter"
no functional changes.

The _NOTE(ARGUNUSED(xxx)) annotations are being exposed to the Visual
Studio 2015 C compiler with the following:

#define _NOTE_ARGUNUSED(...) UNREFERENCED_PARAMETER((__VA_ARGS__));
#define _NOTE(_annotation)   _NOTE_ ## _annotation

Submitted by: Andrew Lee <alee at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18122