drm/amd/display: Add display only once.

[Why]
We call add display multiple times because DESIRED->ENABLED cannot
happen instantaneously. We can't compare the new_state/old_state to
avoid this because on unplug we set the state to DESIRED and on hotplug
the state will still be DESIRED.

[How]
Add a flag to dm_connector_state to keep track of when to enable or
disable HDCP

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>