drm/rcar: call drm_put_dev directly in the ->remove hook

The magic dance drm_platform_exit does is actually a remnant of the
old legacy shadow attach support for platform devices. Modern modesetting
drm drivers shouldn't do this any more (and usb/pci devices actually don't
do this).

Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>