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

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)