fbdev: omap2: panel-dpi: simplify gpio setting

gpiod_set_value_cansleep is a noop when the passed descriptor is NULL.
So there is no need to duplicate the check for NULL; just call the
function unconditionally instead.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>