drm/i915: Apply strongly ordered RCS breadcrumb to gen8/legacy

For legacy ringbuffer mode, we need the new ordered breadcrumb emission
tried and tested on execlists in order to avoid the dreaded "missed
interrupt" syndrome. A secondary advantage of the execlists method is
that it writes to an arbitrary address, useful if one wants to write a
breadcrumb elsewhere.

This fix is taken from commit 7c17d377

 (drm/i915: Use ordered seqno
write interrupt generation on gen8+ execlists).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461932305-14637-1-git-send-email-chris@chris-wilson.co.uk