radeon/vcn: implement encoder dpb management

radeon/vcn: implement encoder dpb management

Previously, the number of previously encoded frames the encoder handled
was 1 - the encoder now supports many more encoded pictures, so the
encoder now has to keep track of multiple reconstructed pictures.

v2: Add a check to make sure an array index is not negative (Boyuan)

Signed-off-by: Thong Thai <thong.thai@amd.com>
Reviewed-by: Boyuan Zhang <boyuan.zhang@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13915>