drm/amd/display: Add trigger connector unplug

[why]
We need a virtual tool that would emulate a physical
connector unplug to usermode, while connector is
still physically plugged in.

[how]
Added a new option to debugfs entry "trigger_hotplug".
It emulates hotplug irq handling scenario by clearing
DC and DM connector states.
It can be triggered with the following command:

	echo 0 > /sys/kernel/debug/dri/0/DP-X/trigger_hotplug

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>