V4L/DVB (8430): videodev: move some functions from v4l2-dev.h to v4l2-common.h or v4l2-ioctl.h

The functions in a header should not belong to another module. The prio functions
belong to v4l2-common.c, so move them to v4l2-common.h.

The ioctl functions belong to v4l2-ioctl.c, so create a new v4l2-ioctl.h header
and move those functions to it.

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