drm/vc4: Fix support for interlaced modes on HDMI.

We really do need to be using the halved V fields.  I had been
confused by the code I was using as a reference because it stored
halved vsync fields but not halved vdisplay, so it looked like I only
needed to divide vdisplay by 2.

This reverts part of Mario's timestamping fixes that prevented
CRTC_HALVE_V from applying, and instead adjusts the timestamping code
to not use the crtc field in that case.

Fixes locking of 1920x1080x60i on my Dell 2408WFP.  There are black
bars on the top and bottom, but I suspect that might be an
under/overscan flags problem as opposed to video timings.

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