drm/amd/display: Fix error in dmesg at boot

[Why]
During DQE's promotion test, error appears in dmesg at boot
on dcn3.1;

[How]
Add NULL pointor check for the pointor to the amdgpu_dm_connector;

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>