drm/i915/gem: Pass mem region to preallocated stolen

As the memory regions are setup early, we can rely on its existence as
we takeover the HW settings from BIOS.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111182143.23479-2-chris@chris-wilson.co.uk