drm/amd/display: skip enable stream on disconnected display

[why]
Virtual signal means there is no display attached.
In this case we will assign a virtual signal type to the stream.
We should only enable the front end of the stream but not the back end.

[how]
When stream is enabling with virtual signal type, skip backend programming.
Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -2718,6 +2718,10 @@ void core_link_enable_stream(
 	enum dc_status status;
 	DC_LOGGER_INIT(pipe_ctx->stream->ctx->logger);
 
+	if (!IS_FPGA_MAXIMUS_DC(core_dc->ctx->dce_environment) &&
+			dc_is_virtual_signal(pipe_ctx->stream->signal))
+		return;
+
 	if (!dc_is_virtual_signal(pipe_ctx->stream->signal)) {
 		stream->link->link_enc->funcs->setup(
 			stream->link->link_enc,
@@ -2860,6 +2864,10 @@ void core_link_disable_stream(struct pipe_ctx *pipe_ctx)
 	struct dc_stream_state *stream = pipe_ctx->stream;
 	struct dc_link *link = stream->sink->link;
 
+	if (!IS_FPGA_MAXIMUS_DC(core_dc->ctx->dce_environment) &&
+			dc_is_virtual_signal(pipe_ctx->stream->signal))
+		return;
+
 #if defined(CONFIG_DRM_AMD_DC_HDCP)
 	update_psp_stream_config(pipe_ctx, true);
 #endif