drm/amd/display: fix multi edp panel instancing

A previous fix attempted to correct mismatch between DM display
targets and dc panel instancing by only counting connected panels.
This behaviour breaks a feature, thus this is an alternative solution
that allows mapping display targets to dc links during mod_power_create.

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>