drm/vc4: plane_state->fb iff plane_state->crtc

Checking both is one too much, so wrap a WARN_ON around it to stope
the copypasta.
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213172612.1514842-7-daniel.vetter@ffwll.ch

drivers/gpu/drm/vc4/vc4_plane.c

@@ -139,7 +139,7 @@ static enum vc4_scaling_mode vc4_get_scaling_mode(u32 src, u32 dst)
 
 static bool plane_enabled(struct drm_plane_state *state)
 {
-	return state->fb && state->crtc;
+	return state->fb && !WARN_ON(!state->crtc);
 }
 
 static struct drm_plane_state *vc4_plane_duplicate_state(struct drm_plane *plane)