fbdev: omap2: improve usage of gpiod API

Since 39b2bbe3 (gpio: add flags argument to gpiod_get*() functions)
which appeared in v3.17-rc1, the gpiod_get* functions take an additional
parameter that allows to specify direction and initial value for output.

Also make use of gpiod_get*_optional where applicable.

Apart from simplification of the affected drivers this is another step
towards making the flags argument to gpiod_get*() mandatory.

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