[media] cobalt: accept unchanged timings when vb2_is_busy()

When vb2_is_busy() it should still be possible to call S_DV_TIMINGS
provided the new timings are the same as the current timings.

For input 1 (test generator) the size is always 1080p, so just return
that.

Fixes a v4l2-compliance issue.

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