[media] mtk-vcodec: fix default OUTPUT buffer size

When calculate OUTPUT buffer size in vidioc_try_fmt, it will
add more size hw need in each plane.
But in mtk_vcodec_enc_set_default_params, it do not add
same size in each plane.
This makes v4l2-compliance test fail.
This patch fix the issue.

Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>