drm/amd/display: turn off the mst hub before we do detection

[why]
not turning off the mst hub before detection on reboot
causes us to not be able to light up displays with mst hook

[how]
on hw init, see if any displays are lit up. if so, turn them off

Signed-off-by: Joseph Gravenor <joseph.gravenor@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>