drm/panel: panel-simple: Add dev_err_probe if backlight could not be found

If the backlight node is not enabled, this (silently) returns with
-EPROBE_DEFER. /sys/kernel/debug/devices_deferred also shows nothing
helpful:
$ cat /sys/kernel/debug/devices_deferred
display

With this patch, there is a helpful hint:
$ cat /sys/kernel/debug/devices_deferred
display panel-simple: Could not find backlight

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220621072118.513346-1-alexander.stein@ew.tq-group.com