drm/i915: Don't dereference request if it may have been retired when printing

This has caught me out on countless occasions, when we retrieve a pointer
from the submission/execlists backend, it does not carry a reference to
the context or ring. Those are only pinned while the request is active,
so if we see the request is already completed, it may be in the process
of being retired and those pointers defunct.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110938
Fixes: 3a068721

 ("drm/i915: Show ring->start for the ELSP context/request queue")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618161951.28820-2-chris@chris-wilson.co.uk