radeon/uvd: use bitstream coded number for symbols of Huffman tables

Signed-off-by: Leo Liu <leo.liu@amd.com>
Fixes: 130d1f45(radeon/uvd: reconstruct MJPEG bitstream)
Cc: "18.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Boyuan Zhang <boyuan.zhang@amd.com>
(cherry picked from commit 3e7b5e5d)