drm/amd/display: Use periodic detection for ipx/headless

drm/amd/display: Use periodic detection for ipx/headless

[WHY]
Hotplug is not detected in headless (no eDP) mode on dcn35x.
With no display dcn35x goes to IPS2 powersaving state where HPD interrupt
is not handled.

[HOW]
Use idle worker thread for periodic detection of HPD in headless mode.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Hung <alex.hung@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>