drm/amd/display: Do not warn NULL dc_sink if forcing connector

[why]
There is a DRM_ERROR when the dc_sink is NULL and
there should not be this warning when the connector
is forced.

[how]
Do not warn if dc_sink is NULL if the connector
is forced.

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>