drm/amd/display: Fix potential null-deref in dm_resume

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Fix potential null-deref in dm_resume [Why] Fixing smatch error: dm_resume() error: we previously assumed 'aconnector->dc_link' could be null [How] Check if dc_link null at the beginning of the loop, so further checks can be dropped.