drm/amd/display: Refactored DC interfaces to support multiple eDP

[Why & How]
Some existing DC interfaces are optimized to return a single eDP
link/stream. Refactored those DC interfaces to support multiple eDP.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>