drm/bridge/sii8620: remove spare CBUS bring-up sequence

In case of MHL3 CBUS is bring-up already in sii8620_got_ecbus_speed.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-11-git-send-email-a.hajda@samsung.com