[media] pwc: Fix pixfmt handling

Before this patch various code in the mode setting patch checked
pdev->pixfmt, but that was not set until the mode setting succeeded, so
it was looking at the old pixfmt! This patch fixes this by making the
pixfmt a parameter to set_video_mode, and setting it from set_video_mode
on success.

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