drm/i915: Register definitions for skylake scalers

Adding register definitions for skylake scalers.
v2:
-add #define for plane selection mask (me)

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>