drm/i915: skylake scaler structure definitions

skylake scaler structure definitions. scalers live in crtc_state as
they are pipe resources. They can be used either as plane scaler or
panel fitter.

scaler assigned to either plane (for plane scaling) or crtc (for panel
fitting) is saved in scaler_id in plane_state or crtc_state respectively.

scaler_id is used instead of scaler pointer in plane or crtc state
to avoid updating scaler pointer everytime a new crtc_state is created.

v2:
-made single copy of min/max values for scalers (Matt)

v3:
-updated commentary for scaler_id (me)

v4:
-converted src/dst ranges to #defines, dropped ratios (Matt)

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>