drm/amd/display: Don't set DSC for phantom pipes

[Description]
Don't set DSC bit for phantom pipes, not
required since phantom pipe don't have
any actual output

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>