drm/i915: Split skl_update_scaler, v4.

commit 2c310b9d2859863826c3688c88218d607d5dd19a
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Mon May 18 12:28:52 2015 +0200

drm/i915: Split skl_update_scaler, v4.

It's easier to read separate functions for crtc and plane scaler state.

Changes since v1:
 - Update documentation.
Changes since v2:
 - Get rid of parameters to skl_update_scaler only used for traces.
   This avoids needing to document the other parameters.
Changes since v3:
 - Rename scaler_idx to scaler_user.

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>