drm/amd/display: Fix for link encoder access for MST.

[Why]
Link encoder in the link could be null for certain links.

[How]
If link encoder in the link is null then get the link encoder
from the stream.
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@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.c

@@ -3305,7 +3305,7 @@ enum dc_status dc_link_allocate_mst_payload(struct pipe_ctx *pipe_ctx)
 {
 	struct dc_stream_state *stream = pipe_ctx->stream;
 	struct dc_link *link = stream->link;
-	struct link_encoder *link_encoder = link->link_enc;
+	struct link_encoder *link_encoder = NULL;
 	struct stream_encoder *stream_encoder = pipe_ctx->stream_res.stream_enc;
 	struct dp_mst_stream_allocation_table proposed_table = {0};
 	struct fixed31_32 avg_time_slots_per_mtp;
@@ -3315,6 +3315,13 @@ enum dc_status dc_link_allocate_mst_payload(struct pipe_ctx *pipe_ctx)
 	enum act_return_status ret;
 	DC_LOGGER_INIT(link->ctx->logger);
 
+	/* Link encoder may have been dynamically assigned to non-physical display endpoint. */
+	if (link->ep_type == DISPLAY_ENDPOINT_PHY)
+		link_encoder = link->link_enc;
+	else if (link->dc->res_pool->funcs->link_encs_assign)
+		link_encoder = link_enc_cfg_get_link_enc_used_by_stream(pipe_ctx->stream->ctx->dc, stream);
+	ASSERT(link_encoder);
+
 	/* enable_link_dp_mst already check link->enabled_stream_count
 	 * and stream is in link->stream[]. This is called during set mode,
 	 * stream_enc is available.
@@ -3392,7 +3399,7 @@ static enum dc_status deallocate_mst_payload(struct pipe_ctx *pipe_ctx)
 {
 	struct dc_stream_state *stream = pipe_ctx->stream;
 	struct dc_link *link = stream->link;
-	struct link_encoder *link_encoder = link->link_enc;
+	struct link_encoder *link_encoder = NULL;
 	struct stream_encoder *stream_encoder = pipe_ctx->stream_res.stream_enc;
 	struct dp_mst_stream_allocation_table proposed_table = {0};
 	struct fixed31_32 avg_time_slots_per_mtp = dc_fixpt_from_int(0);
@@ -3400,6 +3407,13 @@ static enum dc_status deallocate_mst_payload(struct pipe_ctx *pipe_ctx)
 	bool mst_mode = (link->type == dc_connection_mst_branch);
 	DC_LOGGER_INIT(link->ctx->logger);
 
+	/* Link encoder may have been dynamically assigned to non-physical display endpoint. */
+	if (link->ep_type == DISPLAY_ENDPOINT_PHY)
+		link_encoder = link->link_enc;
+	else if (link->dc->res_pool->funcs->link_encs_assign)
+		link_encoder = link_enc_cfg_get_link_enc_used_by_stream(pipe_ctx->stream->ctx->dc, stream);
+	ASSERT(link_encoder);
+
 	/* deallocate_mst_payload is called before disable link. When mode or
 	 * disable/enable monitor, new stream is created which is not in link
 	 * stream[] yet. For this, payload is not allocated yet, so de-alloc