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

Checking both is one too much, so wrap a WARN_ON around it to stope
the copypasta.

Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Brian Starkey <brian.starkey@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213172612.1514842-2-daniel.vetter@ffwll.ch

drivers/gpu/drm/arm/malidp_planes.c

@@ -512,7 +512,7 @@ static int malidp_de_plane_check(struct drm_plane *plane,
 	int i, ret;
 	unsigned int block_w, block_h;
 
-	if (!state->crtc || !state->fb)
+	if (!state->crtc || WARN_ON(!state->fb))
 		return 0;
 
 	fb = state->fb;