drm/omap: fix omap_fbdev_free() when omap_fbdev_create() wasn't called

If we have no crtcs/connectors, fbdev init goes fine, but
omap_fbdev_create() is never called. This means that omap_fbdev->bo is
NULL and omap_fbdev_free() crashes.

Add a check to omap_fbdev_free() to handle the NULL case.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>