ARB prog parser: Fix handling of stateOptModMatNum

The optional array index should clearly be enclosed in square
brackets.  This helps the oglconform test vp_binding.c get a bit
farther, but it still fails.
diff --git a/src/mesa/shader/program_parse.y b/src/mesa/shader/program_parse.y
index e2e83e4..46f3004 100644
--- a/src/mesa/shader/program_parse.y
+++ b/src/mesa/shader/program_parse.y
@@ -1492,9 +1492,9 @@
 	{
 	   $$ = 0;
 	}
-	| stateModMatNum
+	| '[' stateModMatNum ']'
 	{
-	   $$ = $1;
+	   $$ = $2;
 	}
 	;
 stateModMatNum: INTEGER