drm/i915: Emit a request to clear a flushed and idle ring for unbusy bo

In order for bos to retire eventually, a request must be sent down the
ring. This is expected, for example, by occlusion queries for which mesa
will wait upon (whilst running glean) before issuing more batches and so
the normal activity upon the ring is suspended and we need to emit a
request to clear the idle ring.

Reported-by: Jinjin, Wang <jinjin.wang@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30380


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>