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 v5.10.156 by commit 1d160dfb3fdf11ba9447e862c548447f91f4e74a, it was introduced in version v5.9 by commit 5e6c2b4f916157e8f10d093d43e88b2a250d1774. For more details please see the references link.