drm/vc4: Enable precise vblank timestamping for interlaced modes.

On top of the interlaced video mode fix and with some additional
adjustments, this now works well. It has almost the same accuracy
as on regular progressive scan modes.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>