drm/i915: Fix erroneous dereference of batch_obj inside reset_status

drm/i915: Fix erroneous dereference of batch_obj inside reset_status

As the rings may be processed and their requests deallocated in a
different order to the natural retirement during a reset,

/* Whilst this request exists, batch_obj will be on the
 * active_list, and so will hold the active reference. Only when this
 * request is retired will the the batch_obj be moved onto the
 * inactive_list and lose its active reference. Hence we do not need
 * to explicitly hold another reference here.
 */

is violated, and the batch_obj may be dereferenced after it had been
freed on another ring. This can be simply avoided by processing the
status update prior to deallocating any requests.

Fixes regression (a possible OOPS following a GPU hang) from
commit aa60c664e6df502578454621c3a9b1f087ff8d25
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Wed Jun 12 15:13:20 2013 +0300

    drm/i915: find guilty batch buffer on ring resets

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
[danvet: Add the code comment Chris supplied.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>