drm/bridge/sii8620: abstract out sink detection code

MHL1 and MHL3 have different initialization paths. To make both protocols
happy sink detection is put into continuation after link mode enablement.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-14-git-send-email-a.hajda@samsung.com