drm/amdgpu: initialize the vmid_wait with the stub fence

This way we don't need to check for NULL any more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>