V4L/DVB (4365): OVERLAY flag were enabled by mistake

A patch from Aug, 17 activated by mistake OVERLAY flag on cx88-video,
together with several cx88-blackbird cleanups. This makes the cx88 driver
to report:
/dev/video0 [v4l2]: ioctl VIDIOC_G_FBUF: Invalid argument
instead of:
/dev/video0 [v4l2]: no overlay support

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>