[media] vivid: fix format comments

Clarify which formats have an alpha channel and which do not by
using 'x' instead of 'a' if there is no alpha channel.

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