drm/amd/display: Skip accessing array for unknown eng_id

[WHY]
ENGINE_ID_UNKNOWN (-1) is not a valid eng_id and not a valid array
index.

[HOW]
Check whether eng_id is unknown to avoid access array with negative
array index.

This fixes 4 OVERRUN issues reported by Coverity.
Reviewed-by: Harry Wentland <harry.wentland@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/core/dc_link_enc_cfg.c

@@ -248,6 +248,8 @@ static struct link_encoder *get_link_enc_used_by_link(
 
 	for (i = 0; i < MAX_PIPES; i++) {
 		struct link_enc_assignment assignment = state->res_ctx.link_enc_cfg_ctx.link_enc_assignments[i];
+		if (assignment.eng_id == ENGINE_ID_UNKNOWN)
+			continue;
 
 		if (assignment.valid == true && are_ep_ids_equal(&assignment.ep_id, &ep_id))
 			link_enc = link->dc->res_pool->link_encoders[assignment.eng_id - ENGINE_ID_DIGA];
@@ -517,6 +519,8 @@ struct link_encoder *link_enc_cfg_get_link_enc_used_by_link(
 
 	for (i = 0; i < MAX_PIPES; i++) {
 		struct link_enc_assignment assignment = get_assignment(dc, i);
+		if (assignment.eng_id == ENGINE_ID_UNKNOWN)
+			continue;
 
 		if (assignment.valid == true && are_ep_ids_equal(&assignment.ep_id, &ep_id)) {
 			link_enc = link->dc->res_pool->link_encoders[assignment.eng_id - ENGINE_ID_DIGA];
@@ -540,7 +544,7 @@ struct link_encoder *link_enc_cfg_get_next_avail_link_enc(struct dc *dc)
 	for (i = 0; i < MAX_PIPES; i++) {
 		struct link_enc_assignment assignment = get_assignment(dc, i);
 
-		if (assignment.valid)
+		if (assignment.valid && assignment.eng_id != ENGINE_ID_UNKNOWN)
 			encs_assigned[assignment.eng_id - ENGINE_ID_DIGA] = assignment.eng_id;
 	}
 
@@ -602,6 +606,9 @@ struct link_encoder *link_enc_cfg_get_link_enc_used_by_stream_current(
 		struct link_enc_assignment assignment =
 			dc->current_state->res_ctx.link_enc_cfg_ctx.link_enc_assignments[i];
 
+		if (assignment.eng_id == ENGINE_ID_UNKNOWN)
+			continue;
+
 		if (assignment.valid == true && are_ep_ids_equal(&assignment.ep_id, &ep_id)) {
 			link_enc = stream->link->dc->res_pool->link_encoders[assignment.eng_id - ENGINE_ID_DIGA];
 			break;