drm/vmwgfx: fix error return code in vmw_driver_load()

Fix to return -ENOMEM in the fence manager init error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>