lavc/vaapi_hevc: Do not initialize fields twice.

lavc/vaapi_hevc: Do not initialize fields twice.

Fixes the following compilation warnings:
libavcodec/vaapi_hevc.c:155:21: warning: initializer overrides prior initialization of this subobject [-Winitializer-overrides]
        .pic_fields.bits = {
                   ~^~~~
libavcodec/vaapi_hevc.c:125:57: note: previous initialization is here
        .pic_fields.value                             = 0,
                                                        ^
libavcodec/vaapi_hevc.c:175:31: warning: initializer overrides prior initialization of this subobject [-Winitializer-overrides]
        .slice_parsing_fields.bits = {
                             ~^~~~
libavcodec/vaapi_hevc.c:126:57: note: previous initialization is here
        .slice_parsing_fields.value                   = 0,

Reviewed-by: Mark Thompson