drm/client: Fully protect modes[] with dev->mode_config.mutex

In the Linux kernel, the following vulnerability has been resolved: drm/client: Fully protect modes[] with dev->mode_config.mutex The modes[] array contains pointers to modes on the connectors' mode lists, which are protected by dev->mode_config.mutex. Thus we need to extend modes[] the same protection or by the time we use it the elements may already be pointing to freed/reused memory.