drm/i915: Flush read-only buffers from the active list upon idle as well

It is possible for the active list to only contain a read-only buffer so
that the ring->gpu_write_list remains entry. This leads to an
inconsistency between i915_gpu_is_active() and i915_gpu_idle() causing
an infinite spin during the shrinker and an assertion failure that
i915_gpu_idle() does indeed flush all buffers from the active lists.

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