drm/i915: Make scaler_id check in check_crtc_state work for all gens

drm/i915: Make scaler_id check in check_crtc_state work for all gens

During check_crtc_state, scaler_id mispatch is being reported for HSW.
This is applicable for skl+ and not for HSW. It is introduced by
commit id:
    commit a1b2278e4dfcd2dbea85e319ebf73a6b7b2f180b
    Author: Chandra Konduru <chandra.konduru@intel.com>
    Date:   Tue Apr 7 15:28:45 2015 -0700

        drm/i915: skylake panel fitting using shared scalers

This patch will make sure that we leave scaler_id as 0 for platforms
before skl and set for skl+ only. This way scaler_id check during
check_crtc_state will pass for both prior to skl and skl+ platforms.

v2:
-Leave scaler_id as 0 for gen < 9 (Daniel)

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
References: http://lists.freedesktop.org/archives/intel-gfx/2015-May/065741.html
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>