drm/i915/guc: Keep the execbuf client allocated across reset

In order to avoid some complexity in trying to reconstruct the
workqueues across reset, remember them instead. The issue comes when we
have to handle a reset between request allocation and submission, the
request has reserved space in the wq, but is not in any list so we fail
to restore the reserved space. By keeping the execbuf client intact
across the reset, we also keep the reservations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161129121024.22650-5-chris@chris-wilson.co.uk