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>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -6327,7 +6327,12 @@ void dpcd_set_source_specific_data(struct dc_link *link)
 
 void dpcd_update_cable_id(struct dc_link *link)
 {
-	if (!link->link_enc->features.flags.bits.IS_UHBR10_CAPABLE ||
+	struct link_encoder *link_enc = NULL;
+
+	link_enc = link_enc_cfg_get_link_enc(link);
+
+	if (!link_enc ||
+			!link_enc->features.flags.bits.IS_UHBR10_CAPABLE ||
 			link->dprx_status.cable_id_updated)
 		return;