gpiolib: Unify allocation and initialization of GPIO valid mask

Now that the of_gpiochip_add() doesn't use valid mask, we may
unify GPIO valid mask allocation and initialization. With this
it makes a symmetry to the similar which we done for IRQ chip.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>