drm/vmwgfx: Use vmwgfx version 2.18 to signal SM5 compatibility

Signed-off-by: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>
___
v2: Use 2.18 instead of 2.17