drm/bridge/sii8620: fix display of packed pixel modes in MHL2

Currently packed pixel modes in MHL2 can't be displayed. The device
automatically recognizes output format, so setting format other than
RGB causes failure. Fix it by writing proper values to registers.

Tested on MHL1 and MHL2 using various vendors' dongles both in
DVI and HDMI mode.

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/1516706239-9104-1-git-send-email-m.purski@samsung.com