drm/panel: simple: Update calls to gpiod_get*()

Add the new flags argument to calls of (devm_)gpiod_get*() and remove
any direction setting code afterwards.

Currently both forms (with or without the flags argument) are valid
thanks to transitional macros in <linux/gpio/consumer.h>. These macros
will be removed once all consumers are updated and the flags argument
will become compulsary.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>