drm/omap: encoder-tfp410: Convert to the GPIO descriptors API

The GPIO descriptor API is favoured over the plain GPIO API for consumer
drivers. Using it simplifies the driver code.

As the descriptor API handles the active-low flag internally we need to
invert the polarity of all GPIO operations in the driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>