CVE-2025-68286 drm/amd/display: Check NULL before accessing

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Check NULL before accessing WHAT IGT kmscursorlegacy's long-nonblocking-modeset-vs-cursor-atomic fails with NULL pointer dereference. This can be reproduced with both an eDP panel and a DP monitors connected. BUG...