Add a comment about bitdeptharg and inbitdeptharg

Add a comment to vp9_args to point out that bitdeptharg and
inbitdeptharg do not have a corresponding entry in vp9_arg_ctrl_map and
must be listed at the end of vp9_args.

Change-Id: Ic9834ab72599c067156ca5a315824c7f0760824a
1 file changed