drm/i915: Do forcewake reset on gen8

When we get control from BIOS there might be mt forcewake
bits already set. This causes us to do double mt get
without proper clear/ack sequence.

Fix this by clearing mt forcewake register on init,
like we do with older gens.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>