CVE-2025-71138 drm/msm/dpu: Add missing NULL pointer check for pingpong interface

In the Linux kernel, the following vulnerability has been resolved: drm/msm/dpu: Add missing NULL pointer check for pingpong interface It is checked almost always in dpuencoderphyswbsetupctl, but in a single place the check is missing. Also use convenient locals instead of physenc- where availabl...