[media] cobalt: simplify colorspace code

Simplify cobalt_g/try_fmt_vid_cap by not setting the colorspace fields in
pix again (since v4l2_fill_pix_format does that already), and by using
v4l2_fill_mbus_format in cobalt_s_fmt_vid_out which allows the get_fmt
call to be dropped as well.

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