radeon/vcn: Corrected vp9 ref associated data incase of target->codec is NULL

radeon/vcn: Corrected vp9 ref associated data incase of target->codec is NULL

This patch fixes the case where less number of reference surfaces created and destoyed
on need basis. The problem comes when we are refereing old assoiciated data for newly
created target buffer with same address. Here old target buffer destroyed as that
surface is no more used as reference for next frames and when we create a new surface
for the next frame to process we will get the surfaceid and same target address
of destroyed surface.

When new surface/surface->buffer/target ,target->codec is null as we cleared when we
destroy this surface, but per ref_mapping logic, it was taking null associated data
i.e.0 as curr_ref_idx. Hence total reference mapping table goes wrong with wrong data.
Beacuse of this, we have seen corrupted vp9 decoded frames.

Signed-off-by: SureshGuttula <suresh.guttula@amd.corp-partner.google.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5452>