drm crtc_helper: use list_for_each_entry

list_for_each_entry_safe is for walking a list safe against removal
of entries. Here, no entries are removed, so use list_for_each_entry.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>