drm/amd/display: add a debug option to force dp2 lt fallback method

[why]
A debug option is needed to temporarily force dp2 new link training
fallback method for debugging purpose.

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>