drm/i915: Cleanup instdone collection

Consolidate the instdone logic so we can get a bit fancier. This patch also
removes the duplicated print of INSTDONE[0].

v2: (Imre)
- Rebased on top of hangcheck INSTDONE changes.
- Move all INSTDONE registers into a single struct, store it within the
  engine error struct during error capturing.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474379673-28326-1-git-send-email-imre.deak@intel.com