regulator/rpi-panel-attiny: Use the regmap cache

The I2C to the Atmel is very fussy, and locks up easily on
Pi0-3 particularly on reads.

The LCD power status is controlled solely by this driver, so
rather than reading it back from the Atmel, use the regmap cache
to avoid reading values.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Link: https://lore.kernel.org/r/20220124220129.158891-9-detlev.casanova@collabora.com


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