regulator: rpi-panel: Register with a unique backlight name

regulator: rpi-panel: Register with a unique backlight name

There's no reason why 2 Raspberry Pi DSI displays can't be
attached to a Pi Compute Module, so the backlight names need to
be unique.

Use the parent dev_name. It's not as readable, but is unique.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Link: https://lore.kernel.org/r/20220124220129.158891-2-detlev.casanova@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>