radeon/vcn: add and manage render picture list

radeon/vcn: add and manage render picture list

Create a list in decoder to store all render picture buffer pointers that
currently being used in reference picture lists.

During get message buffer call, check each pointer in render_pic_list[]
within given pic->ref[] list, remove pointer that no longer being used by
pic->ref[]. Then add current render surface pointer to the render_pic_list[]
and assign the associated index to result.curr_idx.

As a result, result.curr_idx will have the correct index to represent the
current render picture, instead of the previous increamenting values.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>