backlight: hx8357: Convert to agnostic GPIO API

The of_gpio.h is going to be removed. In preparation of that convert
the driver to the agnostic API.

Fixes: fbbbcd17 ("gpiolib: of: add quirk for locating reset lines with legacy bindings")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20231207161513.3195509-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Lee Jones <lee@kernel.org>