drm/amd/display: Expose internal display flag via debugfs

[Why & How]
Add a per-connector debugfs entry to expose internal display flag,
which is indication that the display is "internally connected"
and not hotpluggable.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>