drm/rockchip: cdn-dp: retry to check sink count

Sometimes the Dock is disconnected, but cdn_dp_encoder_disable is not
triggered by DRM. For example, unplug the Dock in console mode, and
re-plug it again, the cdn_dp_event_work will try to get the sink count
of Dock, since the DP is still active. But the Dock has been powered
down, it need re-power on, and wait for a while until it is ready to
DPCD communication.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>