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: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 00ea881..57efdbe 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -173,6 +173,9 @@
  */
 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);