drm/i915: Split plane updates of crtc->atomic into a helper, v2.

This makes it easier to verify that no changes are done when
calling this from crtc instead.

Changes since v1:
 - Make intel_wm_need_update static and always check it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>