amdgpu: make vamgr per device v2

Each device can have its own vamgr, so make it per device now.
This can fix the failure with multiple GPUs used in one single
process.

v2: rebase

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