drm/rockchip: analogix_dp: make panel detect to an optional action

Some boards don't need to declare a panel device node, like the
display interface is DP monitors, so it's necessary to make the
panel detect to an optional action.

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>