drm/amd/display: fix dc_get_edp_link_panel_inst to only consider links with panels

This function is meant to be used on multi-edp systems and only makes sense
if only links with connected panels are considered.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>