drm/i915: Calculate watermark related members in the crtc_state, v4.

This removes pre/post_wm_update from intel_crtc->atomic, and
creates atomic state for it in intel_crtc.

Changes since v1:
- Rebase on top of wm changes.
Changes since v2:
- Split disable_cxsr into a separate patch.
Changes since v3:
- Move some of the changes to intel_wm_need_update.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56603A49.5000507@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>