drm/amd/display: Update Link Training Fallback logic

[Description]
When CR fails to minimum link rate,
we should reduce lane count to the number lowest cr_done lanes.

[Code Review]
Jun Lei

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>