autogen: add default patch prefix

Signed-off-by: Mihail Konev <k.mvc@ya.ru>

autogen.sh

@@ -9,6 +9,9 @@ cd "$srcdir"
 autoreconf -v --install || exit 1
 cd "$ORIGDIR" || exit $?
 
+git config --local --get format.subjectPrefix >/dev/null 2>&1 ||
+    git config --local format.subjectPrefix "PATCH xf86-video-siliconmotion"
+
 if test -z "$NOCONFIGURE"; then
     exec "$srcdir"/configure "$@"
 fi