gallium radeon/r600/omx/va: Adds support for multiple reference encoding

gallium radeon/r600/omx/va: Adds support for multiple reference encoding

gallium: pipe_h264_enc_picture_desc: ref_idx_lx to ref_idx_lx_list[32], add num_ref_idx_lx_active_minus1
gallium radeon/r600: Change usage of ref_idx_lx to ref_idx_lx_list
gallium omx: Fill out ref_idx_lx_list, num_ref_idx_lx_active_minus1
gallium va: Add support for multiple references encoding

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16286>