drm/amd/display: Add check for headless for idle optimization

drm/amd/display: Add check for headless for idle optimization

[Why]
Currently idle worker thread that checks for HPD while system is in IPS2
only supports headless and static screen use-cases.
In other display-off scenarios hotplug may not work.

[How]
For display-off only allow idle optimization when no display is connected.

Reviewed-by: Sun peng Li <sunpeng.li@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>