drm/vc4: Correct interlaced timings again

The back porch timings were correct, only the sync offset was wrong.
Correct timing is now reported for 1080i and 576i, but the h offset is
incorrect for 480i for non-obvious reasons.

Fixes: c0ba150c "drm/vc4: Correct HDMI timing registers for interlaced modes"
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>