drm/amd/display: Change "error" to "dc_log" at amdgpu_dm dpcd reading stage

[Why]
If reading dpcd happens ahead of hw initialization, then aconnector is NULL
at this point. This is expected, so there is no need to output an error (which will
spam dmesg.log)

[How]
Change type of message from "error" to "DC_LOG_DC".

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>