[media] v4l2-dv-timings: rename v4l2_dv_valid_timings to v4l2_valid_dv_timings

All other functions follow the v4l2_<foo>_dv_timings pattern, do the same for
this function.

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