drm/mediatek: Use CPU when fail to get cmdq event

Even though cmdq client is created successfully, without the cmdq event,
cmdq could not work correctly, so use CPU when fail to get cmdq event.

Fixes: 60fa8c13 ("drm/mediatek: Move gce event property to mutex device node")
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>