drm/amd/display: Initialize writeback connector

[WHAT]
Create a drm_writeback_connector when connector signal equals
SIGNAL_TYPE_VIRTUAL.

Reviewed-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>