drm/i915: Cleanup crt disable sequence on hsw+

drm/i915: Cleanup crt disable sequence on hsw+

Instead of iterating overthe connectors manually, run the last part of
DDI disabling inside the crt post disable function.

This was meant to be addressed before submitting the other commit,
but I missed the review comments.

Fixes: fd6bbda9c7a4 ("drm/i915: Pass crtc_state and connector_state to encoder functions")
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471961888-10771-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[mlankhorst: Fix extra whitespace between functions.]