As pointed by Adrian Bunk, with I2C=m and VIDEO_DEV=y, videodev brokes.

This patch moves the functions that videodev needs from v4l2-common. It also
fixes some Kconfig changes.

After this patch, I2C=m / VIDEO_DEV=y will make v4l2 core statically linked
into kernel. v4l2-common will be m, and all V4L drivers will also be m.

This approach is very conservative, since it is possible to have V4L drivers
that don't need I2C or v4l2-common. The better is to map what drivers really
need v4l2-common, making them to select v4l2-common, and allowing the others to
be 'y', 'm' and 'n'.

Signed-off-by: Mauro Carvalho Chehab