regulator: rpi-panel: Ensure the backlight is off during probe.

The initial state of the Atmel is not defined, so ensure the
backlight PWM is set to 0 by default.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Link: https://lore.kernel.org/r/20220124220129.158891-5-detlev.casanova@collabora.com


Signed-off-by: Mark Brown <broonie@kernel.org>