drm/i915: Use the atomic state in intel_update_primary_planes.

This function was still using the legacy state, convert it to atomic.
While we're at it, fix the FIXME too and disable the primary plane.

v2 (Daniel):
- Add FIXME explaining that update_primary_planes should soon get
  removed anyway.
- Don't call ->disable_plane since we can't disable the primary plane
  with a CS flip (noticed by Ville).

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>