drm/i915: Only record active and pending requests upon a GPU hang

There is no other state pertaining to the completed requests in the
hang, other than gleamed through the ringbuffer, so including the
expired requests in the list of outstanding requests simply adds noise.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-31-git-send-email-chris@chris-wilson.co.uk