util/vma: Fix util_vma_heap_get_max_free_continuous_size calculation

What does this MR do and why?

It was based on misunderstanding of how holes are sorted, they are sorted by address and not by size.

Fixes: df3ba95a ("util/vma: Add function to get max continuous free size")

It's used only by freedreno's replay tool, found the issue when it failed to allocate memory when it clearly should be able to.