drm/bridge/sii8620: Fix link mode selection

Current link mode values do not allow to enable packed pixel modes.

Select packed pixel clock mode, if needed, every time the link mode
register gets updated.

Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1530204243-6370-4-git-send-email-m.purski@samsung.com