OMAPDSS: panel-dpi: use gpiod for enable gpio

The new gpiod API supports automatic handling of active-high/active-low
with DT. To make it possible to use that when booting with DT, change
the panel-dpi's handling of the enable GPIO to use gpiod.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>