[media] pwc: Properly fill all fields on try_fmt

Before this patch the resulting values from a try_fmt were different then
those from a s_fmt with the same parameters. try_fmt simply did not
touch / fill some values like bytesperline at all.

This patch also corrects bytesperline to the proper value for a planar
format such as the YUV420P format the pwc driver produces, which is
the bytesperline value for the biggest plane, rather then those
of all planes added together.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>