media: doc-rst: in the RGB formats table add '-' in undefined bits

In the table of the RGB formats, add an explicit '-' signs
to cells that contain undefined bits.
This makes it more clear how many bits and bytes are used
for each format.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>