[media] vcodec:mediatek: Fix fops_vcodec_release flow for V4L2 Encoder

This patch fix that mtk_vcodec_venc_release should be called after v4l2_m2m_ctx_release

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>