drm/bridge: panel: Ensure backlight is reachable

Ensure that the various options of modules vs builtin results
in being able to call into the backlight code.

https://github.com/raspberrypi/linux/issues/6198



Fixes: 573f8fd0 ("drm/bridge: panel: Name an associated backlight device")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>