drm/i915: Get rid of acthd based guilty batch search

As we seek the guilty batch using request and hangcheck
score, this code is not needed anymore.

v2: Rebase. Passing dev_priv instead of getting it from last_ring

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>