V4L/DVB: video_ioctl2: handle the v4l1 compat bit first and move VIDIOCGMBUF into the switch

Try to make a more sensible sequence of events in __video_do_ioctl: first
check for a valid ops pointer, then get the compat part done. The VIDIOCGMBUF
command is now part of the big switch.

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