amdgpu: fix inefficient vamgr algorithm

issue: UMD allocates top 4GB, but don't do anything, just reserve top 4GB space,
but the performance of VP13 drops from 162fps to 99fps.

root cause:
our va hole list of vamgr is too long by time going.

fix:
reusing old hole as much as possible can make the list shortest.

result:
performance recovers as non-list path, next patch will remove non-list code path.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>