radeonsi/vcn: Allow duplicate buffers in DPB

In case of missing frames (eg. when decoding corrupted streams), there
will be duplicate buffers and all of them needs to be in DPB to keep
the layout correct for decoding.

Cc: mesa-stable
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29055>