drm/i915: only apply GAMMA_MODE IPS WA on HSW

The WA is mentioned in HSW's GAMMA_MODE register documentation, but
not on on BDW's documentation, so let's assume it is not needed there.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>