CVE-2026-68236

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: set new_stream to NULL after release

In dm_update_crtc_state(), the skip_modeset path releases new_stream
via dc_stream_release() but does not set the pointer to NULL.

If a later error (e.g., color management failure) triggers the fail
label, the error path calls dc_stream_release() again on the same
dangling pointer, causing a double release and potential use-after-free.

Fix this by setting new_stream to NULL after the initial release.

(cherry picked from commit 99f3af19073b3ddbfd96e789124cce12c4277b28)