drm/amd/display: Add link enc null ptr check for cable ID (#2597)

[Why]
Certain configurations will result in link encoder
to not be assigned to the link at the time we apply
cable ID logic. We should skip it in those cases.

[How]
Check if link_enc is not null before applying
cable ID.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>