drm/i915: Initialize skylake scalers

Initializing scalers with supported values during crtc init.

v2:
-initialize single copy of min/max values (Matt)

v3:
-moved gen check to callsite (Matt)

v4:
-squashed planes begin with no scaler to here (me)

v5:
-updated init function with updated scaler state structure (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>