drm/amd/display: Only enumerate top local sink as DP2 output

[WHY]
Many DCN generations only have two HPO link encoders and therefore only
support driving a max of two DP2 PHYs. DP2 MST hubs currently can not
pass 3x display validation as each downstream sink is enumerated as
separate DP2 output.

[HOW]
Count MST hubs once by treating only 1st remote sink in topology as an
encoder.

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: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>