backlight: hx8357: Fix potential NULL pointer dereference

The "im" pins are optional. Add missing check in the hx8357_probe().

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/642e1230-3358-4006-a17f-3f297897ae74@moroto.mountain


Fixes: 7d84a63a ("backlight: hx8357: Convert to agnostic GPIO API")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240114143921.550736-1-andriy.shevchenko@linux.intel.com


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