After upgrade to Debian Stable 10 (Buster) this is still a problem. Kernel 4.9.0-9 handle 1920x1080 without problems. Kernel 4.19.0-5 handle max 1024x768. Same hardware, ZOTAC CI547, with Intel HD Graphics 620 using driver i915. In log I see [drm:intel_dp_get_link_train_fallback_values [i915]] *ERROR* Link Training Unsuccessful