drm/omap: Depend on CONFIG_OF

The driver accesses the drm_bridge.of_node field, which is present only
if CONFIG_OF is enabled. As all platforms using omapdrm are OF-based, we
can simply depend on CONFIG_OF.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>