vmwgfx: return an -EFAULT if copy_to_user() fails

copy_to_user() returns the number of bytes remaining to be copied, but
we want to return a negative error code here.  I fixed a couple of these
last year, but I missed this one.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>