drm/panel: simple: Remove unneeded gpiod NULL check

The gpiod API checks for NULL descriptors, so there is no need to
duplicate the check in the driver.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1500249939-8075-1-git-send-email-festevam@gmail.com