[media] pixfmt-007.rst: fix a messed up note in the DCI-P3 doc

The text of the note included text that shouldn't have been part
of the note. Move that out of the note into the proper place.

[mchehab@s-opensource.com: fix a trivial merge conflict]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>