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

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

[ Upstream commit e8b92efa629dac0e70ea4145c5e70616de5f89c8 ]

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
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>