drm/panel: panel-simple: Stash DP AUX bus; allow using it for DDC

If panel-simple is instantiated as a DP AUX bus endpoint then we have
access to the DP AUX bus. Let's stash it in the panel-simple
structure, leaving it NULL for the cases where the panel is
instantiated in other ways.

If we happen to have access to the DP AUX bus and we weren't provided
the ddc-i2c-bus in some other manner, let's use the DP AUX bus for it.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611101711.v10.6.I18e60221f6d048d14d6c50a770b15f356fa75092@changeid