drm/vmwgfx: Fix legacy surface reference size copyback

Surfaces created using the guest-backed surface interface only keeps the
base mip size, so only copy that if the legacy surface reference
ioctl requests the size information.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>