I now booted with amdgpu.dc_log=3D1 and found the following in=
 my log when DC is
on (I'm currently on 4.15.0.36):

[    1.479998] [drm] DC: create_links: connectors_num: physical:4, virtual:0
[    1.480001] [drm] Connector[0] description:signal 32
[    1.480029] [drm] Connector[1] description:signal 4
[    1.480031] [drm] dce110_link_encoder_construct: Failed to get
encoder_cap_info from VBIOS with error code 4!
[    1.480046] [drm] Connector[2] description:signal 2
[    1.480048] [drm] dce110_link_encoder_construct: Failed to get
encoder_cap_info from VBIOS with error code 4!
[    1.480064] [drm] Connector[3] description:signal 2
[    1.480066] [drm] dce110_link_encoder_construct: Failed to get
encoder_cap_info from VBIOS with error code 4!
[    1.492083] [drm] Display Core initialized
[    1.492091] [drm] Display Core initialized!
[    1.492427] [drm] link=3D0, dc_sink_in=3D          (null) is now Disconn=
ected
[    1.492555] [drm] link=3D1, dc_sink_in=3D          (null) is now Disconn=
ected
[    1.492683] [drm] link=3D2, dc_sink_in=3D          (null) is now Disconn=
ected
[    1.505440] [drm] SADs count is: -2, don't need to read it
[    1.505453] [drm] [Detect]   [DDVI][ConnIdx:3] EDID goes here
[    1.505455] [drm] dc_link_detect: EDID decoded goes here
[    1.505456] [drm] link=3D3, dc_sink_in=3D        (ptrval) is now Connect=
ed

This does seem to show that connector 3 is incorrectly identified as dual l=
ink
DVI-D, or at least missing/not using the analogue part.