drm/i915: Calculate correct stolen size for GEN7+

This bug existed in the old code, but was easier to fix here in the
rework. Unfortunately gen7 doesn't have a nice way to figure out the
size and we must use a lookup table.

As Jesse pointed out, there is some confusion in the docs about these
definitions. We're picking the one which seems more accurate, but we
really aren't certain.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>