The fields are deprecated in current vaapi,
setting them to 0 in old versions is fine
as FMO is not implemented.

Fixes the following warnings:
libavcodec/vaapi_h264.c:259:10: warning: 'num_slice_groups_minus1' is deprecated [-Wdeprecated-declarations]
        .num_slice_groups_minus1                    = pps->slice_group_count - 1,
         ^
libavcodec/vaapi_h264.c:260:10: warning: 'slice_group_map_type' is deprecated [-Wdeprecated-declarations]
        .slice_group_map_type                       = pps->mb_slice_group_map_type,
         ^
libavcodec/vaapi_h264.c:261:10: warning: 'slice_group_change_rate_minus1' is deprecated [-Wdeprecated-declarations]
        .slice_group_change_rate_minus1             = 0, /* FMO is not implemented */
         ^

Reviewed-by: Mark Thompson