drm/amd/display: Skip updating link encoder for unknown eng_id

This prevents accessing to negative index of link_encoders array.

This fixes an OVERRUN issue reported by Coverity.
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/link/link_factory.c

@@ -390,7 +390,7 @@ static void link_destruct(struct dc_link *link)
 		 * the dynamic assignment of link encoders to streams. Virtual links
 		 * are not assigned encoder resources on creation.
 		 */
-		if (link->link_id.id != CONNECTOR_ID_VIRTUAL) {
+		if (link->link_id.id != CONNECTOR_ID_VIRTUAL && link->eng_id != ENGINE_ID_UNKNOWN) {
 			link->dc->res_pool->link_encoders[link->eng_id - ENGINE_ID_DIGA] = NULL;
 			link->dc->res_pool->dig_link_enc_count--;
 		}