intel/tools: Disallow control subregisters > 3

> 4 was probably a typo, since the documentation says that there are 4
subregisters (0-3).

Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5514>

src/intel/tools/i965_gram.y

@@ -1853,7 +1853,7 @@ statereg:
 controlreg:
 	CONTROLREG subregnum
 	{
-		if ($2 > 4)
+		if ($2 > 3)
 			error(&@2, "control sub register number %d"
 				   " out of range\n", $2);