drm/amd/display: Create fake sink and stream for writeback connector

[WHAT]
Writeback connectors don't have a physical sink but DC still
needs a sink to function. Create a fake sink and stream for
writeback connectors

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>