amdgpu: add base_preferred parameter to amdgpu_vamgr_find_va

base_preferred parameter is added to amdgpu_vamgr_find_va
so UMD can specify preferred va address when allocating.

Signed-off-by: Ken Wang <Qingqing.Wang@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>