drm/i915: Move init_unused_rings to gem_init_hw

We need to do that every time we resume the rings, not just at load.
I've overlooked this in my untangling of the ring init code.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>