drm: Fix potential null-ptr-deref in drmvblankdestroy_worker()
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.15.80 by commit e884a6c2d49a6c12761e5bed851e9fe93bd923a1, it was introduced in version v5.9 by commit 5e6c2b4f916157e8f10d093d43e88b2a250d1774. For more details please see the references link.