drm/i915/region: fix order when adding blocks

When performing an allocation we try split it down into the largest
possible power-of-two blocks/pages-sizes, and for the common case we
expect to allocate the blocks in descending order. This also naturally
fits with our GTT alignment tricks(including the hugepages selftest),
where we sometimes try to align to the largest possible GTT page-size
for the allocation, in the hope that translates to bigger GTT
page-sizes. Currently, we seem to incorrectly add the blocks in the
opposite order, which is definitely not the intended behaviour.

Reported-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: CQ Tang <cq.tang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201109111249.109365-1-matthew.auld@intel.com