drm/mediatek: Move gce event property to mutex device node

According mtk hardware design, stream_done0 and stream_done1 are
generated by mutex, so we move gce event property to mutex device mode.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>