drm/atmel: 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>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Boris Brezillon <bbrezillon@kernel.org>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213172612.1514842-3-daniel.vetter@ffwll.ch

drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c

@@ -604,7 +604,7 @@ static int atmel_hlcdc_plane_atomic_check(struct drm_plane *p,
 	int ret;
 	int i;
 
-	if (!state->base.crtc || !fb)
+	if (!state->base.crtc || WARN_ON(!fb))
 		return 0;
 
 	crtc_state = drm_atomic_get_existing_crtc_state(s->state, s->crtc);