drm/rockchip: analogix_dp: update the comments about why need to hardcode VOP output mode

The hardware IC designed that VOP must output the RGB10 video format to
eDP contoller, and if eDP panel only support RGB8, then eDP contoller
should cut down the video data, not via VOP contoller, that's why we need
to hardcode the VOP output mode to RGA10 here.

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Tomasz Figa <tomasz.figa@chromium.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>