drm/bridge/sii8620: add MHL3 mode check helper

MHL3 modes differs significantly from MHL1 mode, this helper will be used
frequently to clearly distinguish them.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-3-git-send-email-a.hajda@samsung.com