drm/amd/display: Added local_sink null check before access

[Why & How]
Need to check if local_sink is NULL before accessing.

Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>