drm/rockchip: vop: Don't reject empty modesets

So that when DRM_IOCTL_MODE_SETCRTC is called without a FB nor mode, the
CRTC gets disabled.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Link: http://lkml.kernel.org/g/CAAObsKAv+05ih5U+=4kic_NsjGMhfxYheHR8xXXmacZs+p5SHw@mail.gmail.com