hw/xfree86: Video Driver ABI version 18.0

With the change in the cursor interface in
4c3932620c29c91dfbbc8eb09c84efcaa7ec873e, we need to bump the video
driver ABI number to ensure that drivers are rebuilt to match the new
interface.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>