media: v4l2-dv-timings: add sanity checks for blanking values

Add sanity checks to v4l2_valid_dv_timings() to ensure that the provided
blanking values are reasonable.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: b18787ed

 ([media] v4l2-dv-timings: add new helper module)
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>