drm/amdgpu: fix limiting AV1 to the first instance on VCN3

The job is not yet initialized here.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/2037
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: cdc7893f

 ("drm/amdgpu: use job and ib structures directly in CS parsers")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>