drm/exynos: Replace legacy gpio interface for gpiod interface

Considering the current transition of the GPIO subsystem, remove all
dependencies of the legacy GPIO interface (linux/gpio.h and linux
/of_gpio.h) and replace it with the descriptor-based GPIO approach.

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Inki Dae <inki.dae@samsung.com>