sna: Assert that the CPU bo is not used if the GPU is clear

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_accel.c

@@ -2561,6 +2561,7 @@ use_gpu_bo:
 	return priv->gpu_bo;
 
 use_cpu_bo:
+	assert(!priv->clear);
 	if (priv->cpu_bo == NULL)
 		return NULL;