drm/i915/ringbuffer: Ignore failure to setup the ring on Sandybridge

The ring buffer registers return 0 whilst idle (for some values of idle)
on early Sandybridge hw. Persevere even when all appears hopeless...
Fortunately the head auto-reporting prevents most hangs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31370


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