drm: omapdrm: displays: Remove OF node check in encoder drivers

No encoder is instantiated through platform data anymore, there is no
need to check for OF node presence.

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>