drm/amd/display: add DP2.0 debug option to set MST_EN for SST stream

[why]
Some DP2.0 RX requires us to set MST_EN even for SST configuration.
We added this debug option so we can configure this temporary workaround
for the RX.
Reviewed-by: George Shen <george.shen@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@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

@@ -1869,8 +1869,13 @@ static enum dc_status enable_link_dp(struct dc_state *state,
 		do_fallback = true;
 
 #if defined(CONFIG_DRM_AMD_DC_DCN)
+	/*
+	 * Temporary w/a to get DP2.0 link rates to work with SST.
+	 * TODO DP2.0 - Workaround: Remove w/a if and when the issue is resolved.
+	 */
 	if (dp_get_link_encoding_format(&link_settings) == DP_128b_132b_ENCODING &&
-			pipe_ctx->stream->signal == SIGNAL_TYPE_DISPLAY_PORT) {
+			pipe_ctx->stream->signal == SIGNAL_TYPE_DISPLAY_PORT &&
+			link->dc->debug.set_mst_en_for_sst) {
 		dp_enable_mst_on_sink(link, true);
 	}
 #endif

drivers/gpu/drm/amd/display/dc/dc.h

@@ -664,6 +664,7 @@ struct dc_debug_options {
 #if defined(CONFIG_DRM_AMD_DC_DCN)
 	/* TODO - remove once tested */
 	bool legacy_dp2_lt;
+	bool set_mst_en_for_sst;
 #endif
 	union mem_low_power_enable_options enable_mem_low_power;
 	union root_clock_optimization_options root_clock_optimization;