drm/atomic: Don't try to free a NULL state

Consistently with other free functions, handle the NULL case without
oopsing.

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: NAnder Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic.c

@@ -173,6 +173,9 @@ EXPORT_SYMBOL(drm_atomic_state_clear);
  */
 void drm_atomic_state_free(struct drm_atomic_state *state)
 {
+	if (!state)
+		return;
+
 	drm_atomic_state_clear(state);
 
 	DRM_DEBUG_ATOMIC("Freeing atomic state %p\n", state);