vl: add poc type

vl: add poc type

Different from vce encoding, vcn encoding requires driver side to encode
bitstream header, such as pps, sps and slice header. pic_order_cnt_type
is a required variable when encoding both sps and slice header, therefore
we need to add this new variable here, and hold the value passed from st,
e.g. vaapi interface

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