drm/amd/display: add NULL pointer check

[Why]
In virtual link use case, link->ddc could be NULL.

[How]
Add null pointer check to avoid undefined behavior.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>