drm/vc4: Fix handling of interlaced video modes.

We must not apply CRTC_INTERLACE_HALVE_V to interlaced modes during
mode enumeration, as drm_helper_probe_single_connector_modes
does, so wrap it and reset the effect of CRTC_INTERLACE_HALVE_V
on affected interlaced modes.

Also mode_fixup interlaced modes passed in from user space.

This fixes the vblank timestamping constants and entries in
the mode->crtc_xxx fields needed for precise vblank timestamping.

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