[media] vcodec:mediatek: Refine H264 encoder driver

This patch :
1. remove field and function that unused anymore
2. add support V4L2_MPEG_VIDEO_H264_LEVEL_4_2

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>