drm/amd/display: turn DPMS off on connector unplug

[Why&How]

Set dpms off on the connector that was unplugged, for the side effect of
releasing some references held through deallocation of MST payload. This is
the expected behaviour for non MST devices as well.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>