drm/amdgpu: make the VMID owner always 64bit

Otherwise we could (in theory) run into problems on 32bit systems.

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