[media] vivid: allow s_dv_timings if it is the same as the current

Allow setting the same timings as the current timings (i.e., do nothing in that
case). The code was actually there, but the vb2_is_busy() call was done before
the timings check instead of afterwards.

Found by v4l2-compliance.

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