drm/i915: Walk over encoders in crtc enable/disable using atomic state.

This cleans up another possible use of the connector list,
encoder->crtc is legacy state and should not be used.

With the atomic state as argument it's easy to find the encoder from
the connector it belongs to.

intel_opregion_notify_encoder is a noop for !HAS_DDI, so it's harmless
to unconditionally include it in encoder enable/disable.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470755054-32699-5-git-send-email-maarten.lankhorst@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>