V4L/DVB (5270): Add VIDIOC_G_ENC_INDEX ioctl

The VIDIOC_G_ENC_INDEX ioctl can obtain the MPEG index from an MPEG
encoder.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>