drm-panel: If drm_panel_dp_aux_backlight() fails, don't fail panel probe

If we're using the AUX channel for eDP backlight and it fails to probe
for some reason, let's _not_ fail the panel probe.

At least one case where we could fail to init the backlight is because
of a dead or physically missing panel. As talked about in detail in
the earlier patch in this series, ("drm/panel-edp: If we fail to
powerup/get EDID, use conservative timings"), this can cause the
entire system's display pipeline to fail to come up and that's
non-ideal.

If we fail to init the backlight for some transitory reason, we should
dig in and see if there's a way to fix this (perhaps retries?). Even
in that case, though, having a panel whose backlight is stuck at 100%
(the default, at least in the panel Samsung ATNA33XC20 I tested) is
better than having no panel at all.

Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325145626.3.I552e8af0ddb1691cc0fe5d27ea3d8020e36f7006@changeid