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

Single-threaded forcewake was only used on some early pre-production
ivybridge machines, all the latest ones should use mt forcewake. And
we already assume this in other places of the code (e.g. DERRMR
support in the ddx, or the latest intel_gt_reset patch to reset any
lingering forcewake references left behind by the bios), so don't
bother here, too.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>