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>