vmwgfx: remove useless set memory to zero use memset()

The memory return by kzalloc() or kmem_cache_zalloc() has already
be set to zero, so remove useless memset(0).

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Dave Airlie <airlied@redhat.com>