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