drm/amdgpu: make VMID owner none atomic v2

The variable is protected by the VMID mutex anyway.

v2: grab the mutex while resetting the VMID as well

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>