drm/i915: Single thread force wake isn't used on HSW anymore

Kill the HSW check from the single thread force wake code. HSW
uses MT force wake exclusively these days.

The commit that removed HSW single thread forcewake support:

 commit 36ec8f87
 Author: Daniel Vetter <daniel.vetter@ffwll.ch>
 Date:   Thu Oct 18 14:44:35 2012 +0200

    drm/i915: unconditionally use mt forcewake on hsw/ivb

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>