drm/amd/display: Add comment for preferred_training_settings

[Why]
Currently the naming of preferred_training_settings is ambiguous and has
caused confusion regarding its purpose and usage.

[How]
Add comment to clarify the intention.
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -124,6 +124,10 @@ struct dc_link {
 	struct dc_link_settings cur_link_settings;
 	struct dc_lane_settings cur_lane_setting[LANE_COUNT_DP_MAX];
 	struct dc_link_settings preferred_link_setting;
+	/* preferred_training_settings are override values that
+	 * come from DM. DM is responsible for the memory
+	 * management of the override pointers.
+	 */
 	struct dc_link_training_overrides preferred_training_settings;
 	struct dp_audio_test_data audio_test_data;