drm/rockchip: analogix_dp: allow to work without panel

When the DP output is routed to a external connector there is no
need for a fixed panel, as the panel may be detected via EDID on
the AUX channel. Allow to continue probing if no panel reference
is present.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240621201755.500271-1-l.stach@pengutronix.de