drm/atomic: Don't skip drm_bridge_*() calls if !drm_encoder_helper_funcs

Don't skip drm_bridge_*() calls if encoder->helper_private is NULL.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462982962-10530-5-git-send-email-noralf@tronnes.org