drm/bridge/sii8620: enable MHL3 mode if possible

Since all sub-protocols of MHL3 are already supported MHL3 mode can be
enabled. With this patch it is possible to use packed pixel modes and
clocks up to 300MHz - 1920x1080@60Hz and 4K modes.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-25-git-send-email-a.hajda@samsung.com