drm/amd/display: add interface to query SubVP status

[Why&How]
To enable automated testing through IGT, expose an API that is
accessible through debugfs to query current status of SubVP feature.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>