drm/i915: Move gpu_write_list to per-ring

... to prevent flush processing of an idle (or even absent) ring.

This fixes a regression during suspend from 87acb0a5.

Reported-and-tested-by: Alexey Fisher <bug-track@fisher-privat.net>
Tested-by: Peter Clifton <pcjc2@cam.ac.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>