drm/vmwgfx: Fix nasty write past alloced memory area

commit 0824db38e515644f8d1bfd64adbd7cb2c6ea7c62 upstream.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>