drm/i915: Remove intel_crtc->atomic.disable_ips.

This is a revert of commit 066cf55b "drm/i915: Fix IPS related flicker".
intel_pre_disable_primary already handles this, and now everything
goes through the atomic path there's no need to try to disable ips twice.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454514805-10595-2-git-send-email-maarten.lankhorst@linux.intel.com