The XV601/709 encodings are special: they signal limited range, but use the full range
to encode a larger gamut with R', G' and B' values outside the [0-1] range.

So don't clamp to limited range for these two encodings.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>