drm/virtio: plane_state->fb iff plane_state->crtc

Checking both is one too much, so wrap a WARN_ON around it to stope
the copypasta.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191213172612.1514842-8-daniel.vetter@ffwll.ch
Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -88,7 +88,7 @@ static int virtio_gpu_plane_atomic_check(struct drm_plane *plane,
 	struct drm_crtc_state *crtc_state;
 	int ret;
 
-	if (!state->fb || !state->crtc)
+	if (!state->fb || WARN_ON(!state->crtc))
 		return 0;
 
 	crtc_state = drm_atomic_get_crtc_state(state->state, state->crtc);