V4L/DVB (5970): ivtv: prevent vertical overflow of yuv output

When the video standard is changed, there's no guarantee the framebuffer
dimensions are still legal. The yuv output code uses these dimensions to
calculate the size & position for the video overlay. If the framebuffer
dimensions are now illegal, the output may exceed the vertical limit of the
display, causing distortion.

This patch adds an additional check to ensure the output doesn't exceed
the limits for the current video standard, cropping if required.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>