frontends/va/postproc: Un-break field flag

frontends/va/postproc: Un-break field flag

Fixes an issue where deinterlaced videos would play at half the
framerate, since only one field was repeated, instead of using both
fields. Reverts a change I made previously which broke this.

Fixes: 78786a219ea ("frontends/va: Fix deinterlace bottom field first flag")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3621
Signed-off-by: Thong Thai <thong.thai@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7194>