platform/surface: clean up a variable in surface_dtx_read()

The "&client->ddev->lock" and "&ddev->lock" are the same thing.  Let's
use "&ddev->lock" consistently.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/YF3TgCcpcCYl3a//@mwanda


Signed-off-by: Hans de Goede <hdegoede@redhat.com>