drm/i915: Pin the ring high

If we can use an unmappable ring, try to pin it out of the mappable
aperture. This simple layout preference is to try and keep the mappable
aperture reserved and available to handle GGTT mmapping requests from
userspace without causing evictions and GPU stalls.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180521082131.13744-4-chris@chris-wilson.co.uk