drm/i915: Retire oldest completed request before allocating next

In order to keep the memory allocated for requests reasonably tight, try
to reuse the oldest request (so long as it is completed and has no
external references) for the next allocation.

v2: Throw in a comment to hopefully make sure no one mistakes the
optimistic retirement of the oldest request for simply stealing it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469002875-2335-2-git-send-email-chris@chris-wilson.co.uk