[media] v4l2-dv-timings: mention missing 'reduced blanking V2'

The VESA standard added a version 2 of the reduced blanking formula.
Note in the comment that this is not yet supported by the v4l2_detect_cvt
function. Obviously this should be implemented eventually, but for now add
this as a reminder.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>