drm/gma500: Get rid of duplicate NULL checks

Since GPIOs are optional we don't need to repeat checks that are done
in the GPIO API. Remove them for good.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>