drm/mediatek: update DSI sub driver flow for sending commands to panel

This patch update enable/disable flow of DSI module.
Original flow works on there is a bridge chip: DSI -> bridge -> panel.
In this case: DSI -> panel, the DSI sub driver flow should be updated.
We need to initialize DSI first so that we can send commands to panel.

Signed-off-by: shaoming chen <shaoming.chen@mediatek.com>
Signed-off-by: YT Shen <yt.shen@mediatek.com>
Acked-by: CK Hu <ck.hu@mediatek.com>