drm/amd/display: Check link_index before accessing dc->links[]

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Check link_index before accessing dc->links[] [WHY & HOW] dc->links[] has max size of MAX_LINKS and NULL is return when trying to access with out-of-bound index. This fixes 3 OVERRUN and 1 RESOURCE_LEAK issues reported by Coverity.