backlight: gpio_backlight: Delete pdata inversion

The option to invert the output of the GPIO (active low) is
not used by the only platform still using platform data to
set up a GPIO backlight (one SH board). Delete the option
as we do not expect to expand the use of board files for
this driver, and GPIO descriptors intrinsically keep track
of any signal inversion.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>