drm/amdgpu/vcn: Allow limiting ctx to instance 0 for AV1 at any time

There is no reason to require this to happen on first submitted IB only.
We need to wait for the queue to be idle, but it can be done at any
time (including when there are multiple video sessions active).

Signed-off-by: David Rosca <david.rosca@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>