drm/i915: Double check ring is idle before declaring the GPU wedged

During ring initialisation, sometimes we observe, though not in
production hardware, that the idle flag is not set even though the ring
is empty. Double check before giving up.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>