drm/vc4: Add support for double-clocked modes.

Now that we have infoframes to report the pixel repeat flag, we can
start using it.  Fixes locking the 720x480i and 720x576i modes on my
Dell 2408WFP.  Like the 1920x1080i case, they don't fit properly on
the screen, though.

Signed-off-by: Eric Anholt <eric@anholt.net>