drm/atomic: make release_crtc_commit() static

Fixes the following sparse warning:

drivers/gpu/drm/drm_atomic_helper.c:1360:6: warning:
 symbol 'release_crtc_commit' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170112142157.14684-1-weiyj.lk@gmail.com

drivers/gpu/drm/drm_atomic_helper.c

@@ -1355,7 +1355,7 @@ static int stall_checks(struct drm_crtc *crtc, bool nonblock)
 	return ret < 0 ? ret : 0;
 }
 
-void release_crtc_commit(struct completion *completion)
+static void release_crtc_commit(struct completion *completion)
 {
 	struct drm_crtc_commit *commit = container_of(completion,
 						      typeof(*commit),