drm/amd/display: Avoid updating surface with the same surface under MPO

drm/amd/display: Avoid updating surface with the same surface under MPO

[Why & How]
Although it's dummy updates of surface update for committing stream
updates, we should not have dummy_updates[j].surface all indicating
to the same surface under multiple surfaces case. Otherwise,
copy_surface_update_to_plane() in update_planes_and_stream_state()
will update to the same surface only.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>