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