drm/rockchip: Remove unnecessary NULL check

The expression &private->fbdev_helper can never be NULL, so the check is
completely unnecessary.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170329144401.1804-12-thierry.reding@gmail.com