drm/amd/display: disable FEC while using eDP

[Why]
FEC over eDP support is incomplete.

[How]
Disable FEC over eDP.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Mike Hsieh <chun-wei.hsieh@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Anson Jacob <anson.jacob@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>