drm/mediatek: reduce clear event

No need to clear event again since event always clear before wait.
This fix depend on patch:
  "soc: mediatek: cmdq: add clear option in cmdq_pkt_wfe api"

Fixes: 2f965be7 ("drm/mediatek: apply CMDQ control flow")
Signed-off-by: Dennis YC Hsieh <dennis-yc.hsieh@mediatek.com>
Reviewed-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Acked-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://lore.kernel.org/r/1594136714-11650-10-git-send-email-dennis-yc.hsieh@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>