media: doc: pixfmt-rgb: Make 8 bits per component table more compact

All formats using 8 bits per component can be described with a byte
granularity instead of a bit granularity without loss of precision. This
makes the corresponding table more compact and easier to read.

Signed-off-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>