drm/rockchip: analogix_dp: correct the connector display color format and bpc

Rockchip VOP couldn't output YUV video format for eDP controller, so
when driver detect connector support YUV video format, we need to hack
it down to RGB888.

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>