drm/amdgpu/discovery: add comments about VCN instance handling

Add comments to clarify code that is safe, but triggers and
smatch warning.

Link: https://lists.freedesktop.org/archives/amd-gfx/2022-June/079905.html
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>