util/vma: Add an option to configure high/low preference

The vma_heap allocator was originally designed to prefer high addresses
in order to find bugs in ANV's high address handling.  However, there
are cases where you might want the allocator to prefer lower addresses
for some reason.  This provides a configure bit for exactly this
purpose.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <mesa/mesa!5019>