amdgpu: mostly revert "use the high VA range if possible v2"

This reverts commit 07ea20d5.

Unfortunately it turned out that this change broke some corner cases in
Mesa.

Revert it for now, but keep the high range in separate VA managers.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>