drm/amd/display: Connect DIG FE to its BE before link training starts

[why]
In SST mode no idle pattern will be generated after link training if
DIG FE is not connected to DIG BE.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>