drm/amd/display: Find link encoder for flexible DIG mapping cases

drm/amd/display: Find link encoder for flexible DIG mapping cases

[why & how]
link->link_enc can only be used to identify the link's link encoder
when the link is not permitted to use flexible link encoder
assignments.

Use the correct function for identifying link encoder and add
function pointer guards before calling them.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Signed-off-by: James Lin <pinglei.lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>