drm/amdgpu: limiting AV1 to first instance on VCN4 decode

AV1 is only supported on first instance.

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>