drm/bridge: panel: Name an associated backlight device

Pass the DRM connector name to any configured backlight
device so that userspace can associate the two items.

Ideally this should be in drm_panel, but it is bridge/panel
that creates the drm_connector and therefore knows the name.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

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>