anv: Set up VMA heaps independently from memory heaps

anv: Set up VMA heaps independently from memory heaps

Our VMA allocations are really independent from the memory heaps we
expose via the API.  The only thing that really matters is the GTT size
so we can make the high heap the right size.

Reviewed-by: Ivan Briano <ivan.briano@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>