drm/amd/display: enable audio on DP seamless boot

[Why]
Some external displays with DP can use seamless boot, but their audio endpoints are
only enabled after hotplug.
On boot, these displays inherit UEFI timings without properly initializing audio.

[How]
Enable display audio when seamless booting with an external monitor over DP.
Signed-off-by: Anthony Wang <anthony1.wang@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@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

@@ -3258,6 +3258,16 @@ void core_link_enable_stream(
 		/* Do not touch link on seamless boot optimization. */
 		if (pipe_ctx->stream->apply_seamless_boot_optimization) {
 			pipe_ctx->stream->dpms_off = false;
+
+			/* Still enable stream features & audio on seamless boot for DP external displays */
+			if (pipe_ctx->stream->signal == SIGNAL_TYPE_DISPLAY_PORT) {
+				enable_stream_features(pipe_ctx);
+				if (pipe_ctx->stream_res.audio != NULL) {
+					pipe_ctx->stream_res.stream_enc->funcs->dp_audio_enable(pipe_ctx->stream_res.stream_enc);
+					dc->hwss.enable_audio_stream(pipe_ctx);
+				}
+			}
+
 #if defined(CONFIG_DRM_AMD_DC_HDCP)
 			update_psp_stream_config(pipe_ctx, false);
 #endif