drm/i915: Allocate rings from stolen

If we have stolen available, make use of it for ringbuffer allocation.
Previously this was restricted to !llc platforms, as writing to stolen
requires a GGTT mapping - but now that we have partial mappable support,
the mappable aperture isn't quite so precious so we can use it more
freely and ringbuffers are a good user for the otherwise wasted stolen.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-18-chris@chris-wilson.co.uk